feat(unit-3): Layout & Navigation — sidebar, mobile overlay, theme toggle

- Remove Topbar; move UserMenu + LanguageSwitcher to sidebar footer
- Add ThemeToggle (dark/light) with localStorage persistence
- Add useTheme hook; no-flash inline script in index.html
- Add MobileBar (hamburger + app name, mobile-only)
- Add SidebarOverlay (slide-in from left, backdrop closes it)
- Sidebar: role-filtered nav (BR-U3-01–06), onClose prop for mobile
- AppLayout: desktop sidebar-only layout, mobile bar + overlay
- i18n: theme.* and nav.openMenu/closeMenu keys (NL + EN)
- Tests: 43/43 passing (14 new — useTheme, Sidebar roles, AppLayout)

Stories: US-08, US-18, US-19

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.tsx

@@ -1,17 +1,14 @@
-import { useEffect } from 'react';
+import { useState, useEffect } from 'react';
 import { Outlet, useNavigate } from '@tanstack/react-router';
 import { useAuth } from '@/contexts/auth-context';
 import { Sidebar } from '@/components/layout/Sidebar';
-import { Topbar } from '@/components/layout/Topbar';
+import { MobileBar } from '@/components/layout/MobileBar';
+import { SidebarOverlay } from '@/components/layout/SidebarOverlay';
 
-/**
- * Shell for authenticated routes. Renders the persistent chrome and reacts to
- * runtime auth loss (e.g. a failed refresh during an API call) by redirecting
- * to /login (BR-U1-14).
- */
 export function AppLayout() {
     const { isAuthenticated, status } = useAuth();
     const navigate = useNavigate();
+    const [isMenuOpen, setIsMenuOpen] = useState(false);
 
     useEffect(() => {
         if (status === 'guest') {
@@ -25,9 +22,12 @@ export function AppLayout() {
 
     return (
         <div className="flex min-h-svh">
-            <Sidebar />
+            <div className="hidden md:flex">
+                <Sidebar />
+            </div>
+            {isMenuOpen && <SidebarOverlay onClose={() => setIsMenuOpen(false)} />}
             <div className="flex flex-1 flex-col">
-                <Topbar />
+                <MobileBar onMenuOpen={() => setIsMenuOpen(true)} />
                 <main className="flex-1 p-6" data-testid="app-main">
                     <Outlet />
                 </main>