feat(frontend): Implement navigation and layout (#44)

Implements the main navigation and layout structure:

- Sidebar component with collapsible navigation and keyboard shortcut
- AppHeader with project switcher and user menu
- AppBreadcrumbs with auto-generation from pathname
- ProjectSwitcher dropdown for quick project navigation
- UserMenu with profile, settings, and logout
- AppLayout component combining all layout elements

Features:
- Responsive design (mobile sidebar sheet, desktop sidebar)
- Keyboard navigation (Cmd/Ctrl+B to toggle sidebar)
- Dark mode support
- WCAG AA accessible (ARIA labels, focus management)

All 125 tests passing. Follows design system guidelines.

frontend/src/components/layout/index.ts

@@ -3,7 +3,17 @@
  * Common layout elements for authenticated pages
  */
 
+// Existing components
 export { Header } from './Header';
 export { Footer } from './Footer';
 export { HeaderSkeleton } from './HeaderSkeleton';
 export { AuthLoadingSkeleton } from './AuthLoadingSkeleton';
+
+// Application layout components (Issue #44)
+export { AppLayout, PageContainer, PageHeader } from './AppLayout';
+export { AppHeader } from './AppHeader';
+export { Sidebar } from './Sidebar';
+export { AppBreadcrumbs } from './AppBreadcrumbs';
+export type { BreadcrumbItem } from './AppBreadcrumbs';
+export { ProjectSwitcher, ProjectSelect } from './ProjectSwitcher';
+export { UserMenu } from './UserMenu';