Add Docker Compose project scaffold

Sets up the full monorepo structure with:
- Multi-stage Dockerfile (client-build + production stages)
- docker-compose.yml for production, docker-compose.dev.yml overlay for development
- Express server (port 3000) with pg pool, health route, and SPA static file serving
- React 18 + Vite client with react-router-dom v6, nav bar, and placeholder page components
- .env.example with PostgreSQL and app config
- Empty db/migrations/ directory for future migrations
- CLAUDE.md updated with development workflow commands

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/App.jsx

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+import { Routes, Route, NavLink } from 'react-router-dom';
+import PaycheckView from './pages/PaycheckView.jsx';
+import Bills from './pages/Bills.jsx';
+import Settings from './pages/Settings.jsx';
+import MonthlySummary from './pages/MonthlySummary.jsx';
+import AnnualOverview from './pages/AnnualOverview.jsx';
+
+function App() {
+  return (
+    <div>
+      <nav style={{ display: 'flex', gap: '1rem', padding: '1rem', borderBottom: '1px solid #ccc' }}>
+        <NavLink to="/">Paycheck</NavLink>
+        <NavLink to="/bills">Bills</NavLink>
+        <NavLink to="/settings">Settings</NavLink>
+        <NavLink to="/summary/monthly">Monthly Summary</NavLink>
+        <NavLink to="/summary/annual">Annual Overview</NavLink>
+      </nav>
+
+      <main style={{ padding: '1rem' }}>
+        <Routes>
+          <Route path="/" element={<PaycheckView />} />
+          <Route path="/bills" element={<Bills />} />
+          <Route path="/settings" element={<Settings />} />
+          <Route path="/summary/monthly" element={<MonthlySummary />} />
+          <Route path="/summary/annual" element={<AnnualOverview />} />
+        </Routes>
+      </main>
+    </div>
+  );
+}
+
+export default App;

client/src/main.jsx

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+import React from 'react';
+import ReactDOM from 'react-dom/client';
+import { BrowserRouter } from 'react-router-dom';
+import App from './App.jsx';
+
+ReactDOM.createRoot(document.getElementById('root')).render(
+  <React.StrictMode>
+    <BrowserRouter>
+      <App />
+    </BrowserRouter>
+  </React.StrictMode>
+);

client/src/pages/AnnualOverview.jsx

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+function AnnualOverview() {
+  return <div><h1>Annual Overview</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default AnnualOverview;

client/src/pages/Bills.jsx

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+function Bills() {
+  return <div><h1>Bills</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default Bills;

client/src/pages/MonthlySummary.jsx

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+function MonthlySummary() {
+  return <div><h1>Monthly Summary</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default MonthlySummary;

client/src/pages/PaycheckView.jsx

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+function PaycheckView() {
+  return <div><h1>Paycheck View</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default PaycheckView;

client/src/pages/Settings.jsx

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+function Settings() {
+  return <div><h1>Settings</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default Settings;