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/i18n/locales/en/translation.json

@@ -11,7 +11,13 @@
         "users": "Users",
         "cms": "CMS",
         "profile": "Profile",
-        "settings": "Settings"
+        "settings": "Settings",
+        "openMenu": "Open navigation",
+        "closeMenu": "Close navigation"
+    },
+    "theme": {
+        "switchToLight": "Switch to light theme",
+        "switchToDark": "Switch to dark theme"
     },
     "login": {
         "title": "Sign in",

frontend/src/i18n/locales/nl/translation.json

@@ -11,7 +11,13 @@
         "users": "Gebruikers",
         "cms": "CMS",
         "profile": "Profiel",
-        "settings": "Instellingen"
+        "settings": "Instellingen",
+        "openMenu": "Navigatie openen",
+        "closeMenu": "Navigatie sluiten"
+    },
+    "theme": {
+        "switchToLight": "Overschakelen naar licht thema",
+        "switchToDark": "Overschakelen naar donker thema"
     },
     "login": {
         "title": "Inloggen",