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/store/themeStore.ts

@@ -0,0 +1,53 @@
+import { create } from 'zustand'
+import { persist } from 'zustand/middleware'
+
+type Theme = 'light' | 'dark' | 'system'
+
+interface ThemeState {
+  theme: Theme
+  resolvedTheme: 'light' | 'dark'
+  setTheme: (theme: Theme) => void
+}
+
+const getSystemTheme = (): 'light' | 'dark' => {
+  if (typeof window === 'undefined') return 'light'
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+}
+
+const applyTheme = (theme: Theme): 'light' | 'dark' => {
+  const resolved = theme === 'system' ? getSystemTheme() : theme
+  const root = document.documentElement
+
+  if (resolved === 'dark') {
+    root.classList.add('dark')
+  } else {
+    root.classList.remove('dark')
+  }
+
+  return resolved
+}
+
+export const useThemeStore = create<ThemeState>()(
+  persist(
+    (set) => ({
+      theme: 'system',
+      resolvedTheme: getSystemTheme(),
+
+      setTheme: (theme: Theme) => {
+        const resolvedTheme = applyTheme(theme)
+        set({ theme, resolvedTheme })
+      },
+    }),
+    {
+      name: 'theme-storage',
+      onRehydrateStorage: () => (state) => {
+        // Apply theme on initial load after rehydration
+        if (state) {
+          applyTheme(state.theme)
+        }
+      },
+    }
+  )
+)
+
+export default useThemeStore