diff --git a/src/app/globals.css b/src/app/globals.css
new file mode 100644
index 0000000..17f55e5
--- /dev/null
+++ b/src/app/globals.css
@@ -0,0 +1,226 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+/* ─── Base ─────────────────────────────────────────────────────────────────── */
+
+:root {
+  --color-positive: #059669;
+  --color-negative: #dc2626;
+  --color-interest: #f59e0b;
+  --color-capital: #2563eb;
+  --color-insurance: #8b5cf6;
+  --color-charges: #6b7280;
+  --color-rent: #ef4444;
+  --color-projected: #2563eb;
+}
+
+html {
+  scroll-behavior: smooth;
+}
+
+body {
+  @apply bg-slate-50 text-slate-900 antialiased;
+  font-feature-settings: 'cv02', 'cv03', 'cv04', 'cv11';
+}
+
+.dark body,
+html.dark body {
+  @apply bg-slate-950 text-slate-100;
+}
+
+/* ─── Form Elements ────────────────────────────────────────────────────────── */
+
+input[type='number']::-webkit-inner-spin-button,
+input[type='number']::-webkit-outer-spin-button {
+  -webkit-appearance: none;
+  margin: 0;
+}
+
+input[type='number'] {
+  -moz-appearance: textfield;
+}
+
+/* ─── Cards ────────────────────────────────────────────────────────────────── */
+
+@layer components {
+  .card {
+    @apply bg-white rounded-xl border border-slate-200 shadow-sm
+           dark:bg-slate-900 dark:border-slate-800;
+  }
+
+  .card-header {
+    @apply flex items-center justify-between px-5 py-4 cursor-pointer select-none
+           border-b border-slate-100 hover:bg-slate-50/50 transition-colors
+           dark:border-slate-800 dark:hover:bg-slate-800/50;
+  }
+
+  .card-body {
+    @apply px-5 py-4;
+  }
+
+  /* Form field */
+  .form-field {
+    @apply flex flex-col gap-1;
+  }
+
+  .form-label {
+    @apply text-xs font-medium text-slate-500 uppercase tracking-wide
+           dark:text-slate-400;
+  }
+
+  .form-input {
+    @apply block w-full rounded-lg border-slate-300 text-sm
+           focus:border-blue-500 focus:ring-blue-500
+           placeholder:text-slate-400 transition-colors
+           dark:bg-slate-800 dark:border-slate-700 dark:text-slate-100
+           dark:placeholder:text-slate-500 dark:focus:border-blue-400 dark:focus:ring-blue-400;
+  }
+
+  .form-input-sm {
+    @apply form-input py-1.5 px-2.5 text-sm;
+  }
+
+  /* Metric cards */
+  .metric-card {
+    @apply bg-white rounded-xl border border-slate-200 p-4 shadow-sm
+           dark:bg-slate-900 dark:border-slate-800;
+  }
+
+  .metric-value {
+    @apply text-2xl font-bold text-slate-900 tabular-nums
+           dark:text-slate-100;
+  }
+
+  .metric-label {
+    @apply text-xs font-medium text-slate-500 mt-1
+           dark:text-slate-400;
+  }
+
+  /* Table */
+  .data-table {
+    @apply w-full text-sm;
+  }
+
+  .data-table thead th {
+    @apply px-3 py-2.5 text-left text-xs font-semibold text-slate-500
+           uppercase tracking-wider bg-slate-50 border-b border-slate-200
+           whitespace-nowrap
+           dark:bg-slate-800/50 dark:text-slate-400 dark:border-slate-700;
+  }
+
+  .data-table tbody td {
+    @apply px-3 py-2 tabular-nums border-b border-slate-100
+           dark:border-slate-800;
+  }
+
+  .data-table tbody tr:hover {
+    @apply bg-blue-50/40 dark:bg-blue-950/30;
+  }
+
+  /* Tab styles */
+  .tab-btn {
+    @apply px-4 py-2.5 text-sm font-medium rounded-lg transition-all
+           text-slate-500 hover:text-slate-700 hover:bg-slate-100
+           dark:text-slate-400 dark:hover:text-slate-200 dark:hover:bg-slate-800;
+  }
+
+  .tab-btn-active {
+    @apply bg-white text-blue-700 shadow-sm border border-slate-200
+           dark:bg-slate-800 dark:text-blue-400 dark:border-slate-700;
+  }
+
+  /* Badge */
+  .badge {
+    @apply inline-flex items-center px-2 py-0.5 text-xs font-medium rounded-full;
+  }
+
+  .badge-blue {
+    @apply badge bg-blue-50 text-blue-700
+           dark:bg-blue-950/50 dark:text-blue-400;
+  }
+
+  .badge-green {
+    @apply badge bg-emerald-50 text-emerald-700
+           dark:bg-emerald-950/50 dark:text-emerald-400;
+  }
+
+  .badge-amber {
+    @apply badge bg-amber-50 text-amber-700
+           dark:bg-amber-950/50 dark:text-amber-400;
+  }
+
+  .badge-red {
+    @apply badge bg-red-50 text-red-700
+           dark:bg-red-950/50 dark:text-red-400;
+  }
+}
+
+/* ─── Animations ───────────────────────────────────────────────────────────── */
+
+@layer utilities {
+  .animate-collapsible-open {
+    animation: collapsible-open 0.25s ease-out;
+  }
+
+  .animate-collapsible-close {
+    animation: collapsible-close 0.2s ease-in forwards;
+  }
+}
+
+@keyframes collapsible-open {
+  from {
+    opacity: 0;
+    max-height: 0;
+  }
+  to {
+    opacity: 1;
+    max-height: 1000px;
+  }
+}
+
+@keyframes collapsible-close {
+  from {
+    opacity: 1;
+    max-height: 1000px;
+  }
+  to {
+    opacity: 0;
+    max-height: 0;
+  }
+}
+
+/* ─── Print ────────────────────────────────────────────────────────────────── */
+
+@media print {
+  .no-print {
+    display: none !important;
+  }
+
+  body {
+    background: white;
+    color: black;
+  }
+
+  html.dark body {
+    background: white;
+    color: black;
+  }
+
+  .card {
+    break-inside: avoid;
+    box-shadow: none;
+    border: 1px solid #e2e8f0;
+    background: white;
+  }
+}
+
+/* ─── Dark mode utility overrides ──────────────────────────────────────────── */
+
+html.dark select {
+  @apply bg-slate-800 border-slate-700 text-slate-100;
+}
+
+html.dark input[type='checkbox'] {
+  @apply bg-slate-800 border-slate-600;
+}
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
new file mode 100644
index 0000000..998c42c
--- /dev/null
+++ b/src/app/layout.tsx
@@ -0,0 +1,43 @@
+import type { Metadata } from 'next';
+import { Inter } from 'next/font/google';
+import './globals.css';
+
+const inter = Inter({
+  subsets: ['latin'],
+  display: 'swap',
+  variable: '--font-inter',
+});
+
+export const metadata: Metadata = {
+  title: 'SimO — Simulateur de Prêt Immobilier',
+  description:
+    'Simulez votre prêt immobilier gratuitement : PTZ, Action Logement, Primo Boost, amortissement détaillé, comparaison loyer/crédit et projection de revente.',
+  keywords: 'simulateur, prêt immobilier, PTZ, Action Logement, amortissement, crédit immobilier',
+  openGraph: {
+    title: 'SimO — Simulateur de Prêt Immobilier',
+    description: 'Simulez votre prêt immobilier gratuitement avec tous les dispositifs aidés.',
+    type: 'website',
+  },
+};
+
+export default function RootLayout({ children }: { children: React.ReactNode }) {
+  // Inline script to prevent FOUC (flash of unstyled content) on dark mode
+  const themeScript = `
+    (function() {
+      try {
+        var t = localStorage.getItem('simo-theme') || 'system';
+        var dark = t === 'dark' || (t === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+        if (dark) document.documentElement.classList.add('dark');
+      } catch(e) {}
+    })();
+  `;
+
+  return (
+    <html lang="fr" className={inter.variable} suppressHydrationWarning>
+      <head>
+        <script dangerouslySetInnerHTML={{ __html: themeScript }} />
+      </head>
+      <body className={inter.className}>{children}</body>
+    </html>
+  );
+}
diff --git a/src/components/ui.tsx b/src/components/ui.tsx
new file mode 100644
index 0000000..4316c4d
--- /dev/null
+++ b/src/components/ui.tsx
@@ -0,0 +1,241 @@
+'use client';
+
+import React, { useState, type ReactNode } from 'react';
+
+// ─── Collapsible Section ────────────────────────────────────────────────────
+
+interface CollapsibleSectionProps {
+  title: string;
+  icon?: ReactNode;
+  defaultOpen?: boolean;
+  badge?: ReactNode;
+  children: ReactNode;
+}
+
+export function CollapsibleSection({
+  title,
+  icon,
+  defaultOpen = true,
+  badge,
+  children,
+}: CollapsibleSectionProps) {
+  const [open, setOpen] = useState(defaultOpen);
+
+  return (
+    <div className="card">
+      <button
+        className="card-header w-full"
+        onClick={() => setOpen(!open)}
+        aria-expanded={open}
+      >
+        <div className="flex items-center gap-2">
+          {icon && <span className="text-slate-400">{icon}</span>}
+          <h3 className="text-sm font-semibold text-slate-700 dark:text-slate-200">{title}</h3>
+          {badge}
+        </div>
+        <svg
+          className={`w-4 h-4 text-slate-400 transition-transform duration-200 ${
+            open ? 'rotate-180' : ''
+          }`}
+          fill="none"
+          viewBox="0 0 24 24"
+          stroke="currentColor"
+          strokeWidth={2}
+        >
+          <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
+        </svg>
+      </button>
+      {open && <div className="card-body animate-fade-in">{children}</div>}
+    </div>
+  );
+}
+
+// ─── Number Input ───────────────────────────────────────────────────────────
+
+interface NumberInputProps {
+  label: string;
+  value: number;
+  onChange: (v: number) => void;
+  suffix?: string;
+  prefix?: string;
+  min?: number;
+  max?: number;
+  step?: number;
+  hint?: string;
+  className?: string;
+}
+
+export function NumberInput({
+  label,
+  value,
+  onChange,
+  suffix,
+  prefix,
+  min,
+  max,
+  step = 1,
+  hint,
+  className,
+}: NumberInputProps) {
+  return (
+    <div className={`form-field ${className ?? ''}`}>
+      <label className="form-label">{label}</label>
+      <div className="relative">
+        {prefix && (
+          <span className="absolute left-3 top-1/2 -translate-y-1/2 text-sm text-slate-400 pointer-events-none">
+            {prefix}
+          </span>
+        )}
+        <input
+          type="number"
+          className={`form-input-sm ${prefix ? 'pl-7' : ''} ${suffix ? 'pr-10' : ''}`}
+          value={value}
+          onChange={(e) => onChange(parseFloat(e.target.value) || 0)}
+          min={min}
+          max={max}
+          step={step}
+        />
+        {suffix && (
+          <span className="absolute right-3 top-1/2 -translate-y-1/2 text-sm text-slate-400 pointer-events-none">
+            {suffix}
+          </span>
+        )}
+      </div>
+      {hint && <p className="text-xs text-slate-400 mt-0.5">{hint}</p>}
+    </div>
+  );
+}
+
+// ─── Rate Input (special for percentages) ───────────────────────────────────
+
+interface RateInputProps {
+  label: string;
+  value: number; // stored as decimal (0.0325 for 3.25%)
+  onChange: (v: number) => void;
+  hint?: string;
+  className?: string;
+}
+
+export function RateInput({ label, value, onChange, hint, className }: RateInputProps) {
+  const displayValue = Math.round(value * 10000) / 100; // 0.0325 → 3.25
+  return (
+    <div className={`form-field ${className ?? ''}`}>
+      <label className="form-label">{label}</label>
+      <div className="relative">
+        <input
+          type="number"
+          className="form-input-sm pr-8"
+          value={displayValue}
+          onChange={(e) => onChange((parseFloat(e.target.value) || 0) / 100)}
+          step={0.01}
+          min={0}
+          max={100}
+        />
+        <span className="absolute right-3 top-1/2 -translate-y-1/2 text-sm text-slate-400 pointer-events-none">
+          %
+        </span>
+      </div>
+      {hint && <p className="text-xs text-slate-400 mt-0.5">{hint}</p>}
+    </div>
+  );
+}
+
+// ─── Toggle ─────────────────────────────────────────────────────────────────
+
+interface ToggleProps {
+  options: { value: string; label: string }[];
+  value: string;
+  onChange: (v: string) => void;
+  className?: string;
+}
+
+export function Toggle({ options, value, onChange, className }: ToggleProps) {
+  return (
+    <div className={`inline-flex rounded-lg bg-slate-100 dark:bg-slate-800 p-0.5 ${className ?? ''}`}>
+      {options.map((opt) => (
+        <button
+          key={opt.value}
+          className={`px-4 py-1.5 text-sm font-medium rounded-md transition-all ${
+            value === opt.value
+              ? 'bg-white dark:bg-slate-700 text-blue-700 dark:text-blue-400 shadow-sm'
+              : 'text-slate-500 dark:text-slate-400 hover:text-slate-700 dark:hover:text-slate-200'
+          }`}
+          onClick={() => onChange(opt.value)}
+        >
+          {opt.label}
+        </button>
+      ))}
+    </div>
+  );
+}
+
+// ─── Tabs ───────────────────────────────────────────────────────────────────
+
+interface TabsProps {
+  tabs: { id: string; label: string; icon?: ReactNode }[];
+  activeTab: string;
+  onTabChange: (id: string) => void;
+}
+
+export function Tabs({ tabs, activeTab, onTabChange }: TabsProps) {
+  return (
+    <div className="flex gap-1 p-1 bg-slate-100 dark:bg-slate-800/50 rounded-xl overflow-x-auto">
+      {tabs.map((tab) => (
+        <button
+          key={tab.id}
+          className={`tab-btn whitespace-nowrap flex items-center gap-1.5 ${
+            activeTab === tab.id ? 'tab-btn-active' : ''
+          }`}
+          onClick={() => onTabChange(tab.id)}
+        >
+          {tab.icon}
+          {tab.label}
+        </button>
+      ))}
+    </div>
+  );
+}
+
+// ─── Tooltip ────────────────────────────────────────────────────────────────
+
+interface TooltipProps {
+  text: string;
+  children: ReactNode;
+}
+
+export function Tooltip({ text, children }: TooltipProps) {
+  return (
+    <span className="relative group">
+      {children}
+      <span
+        className="absolute bottom-full left-1/2 -translate-x-1/2 mb-2 px-3 py-1.5
+                   bg-slate-800 text-white text-xs rounded-lg whitespace-nowrap
+                   opacity-0 invisible group-hover:opacity-100 group-hover:visible
+                   transition-all duration-200 pointer-events-none z-50
+                   after:content-[''] after:absolute after:top-full after:left-1/2
+                   after:-translate-x-1/2 after:border-4 after:border-transparent
+                   after:border-t-slate-800"
+      >
+        {text}
+      </span>
+    </span>
+  );
+}
+
+// ─── Empty State ────────────────────────────────────────────────────────────
+
+interface EmptyStateProps {
+  title: string;
+  description: string;
+  action?: ReactNode;
+}
+
+export function EmptyState({ title, description, action }: EmptyStateProps) {
+  return (
+    <div className="text-center py-12 px-4">
+      <p className="text-sm font-medium text-slate-900 dark:text-slate-100">{title}</p>
+      <p className="text-sm text-slate-500 dark:text-slate-400 mt-1">{description}</p>
+      {action && <div className="mt-4">{action}</div>}
+    </div>
+  );
+}