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/App.tsx

@@ -2,9 +2,11 @@ import { useEffect } from 'react'
 import { RouterProvider } from 'react-router-dom'
 import { router } from '@/router'
 import { useAuthStore } from '@/store/authStore'
+import { useThemeStore } from '@/store/themeStore'
 
 function App() {
   const { isAuthenticated, fetchUser, setLoading } = useAuthStore()
+  const { theme, setTheme } = useThemeStore()
 
   useEffect(() => {
     // On app load, check if we have a token and fetch user data
@@ -18,6 +20,19 @@ function App() {
     }
   }, [])
 
+  // Listen for system theme changes
+  useEffect(() => {
+    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)')
+    const handleChange = () => {
+      if (theme === 'system') {
+        setTheme('system') // Re-apply to update resolvedTheme
+      }
+    }
+
+    mediaQuery.addEventListener('change', handleChange)
+    return () => mediaQuery.removeEventListener('change', handleChange)
+  }, [theme, setTheme])
+
   return <RouterProvider router={router} />
 }