Add dark mode, export preview, and keyboard navigation

- Add theme store with light/dark/system modes and ThemeToggle component
- Prevent flash of wrong theme on initial load via inline script
- Add ExportPreviewModal for previewing session exports before download
- Add copy-to-clipboard functionality to session export
- Implement keyboard shortcuts for tree navigation (1-9 options, Esc back, Enter continue)
- Display keyboard hints in tree navigation UI
- Fix findNode to safely handle undefined structure parameter
- Update page title to "Apoklisis"

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

frontend/src/components/layout/AppLayout.tsx

@@ -1,5 +1,6 @@
 import { Link, useLocation, useNavigate, Outlet } from 'react-router-dom'
 import { useAuthStore } from '@/store/authStore'
+import { ThemeToggle } from '@/components/common/ThemeToggle'
 import { cn } from '@/lib/utils'
 
 export function AppLayout() {
@@ -48,6 +49,7 @@ export function AppLayout() {
             <span className="hidden text-sm text-muted-foreground sm:block">
               {user?.name || user?.email}
             </span>
+            <ThemeToggle />
             <button
               onClick={handleLogout}
               className={cn(