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/hooks/useKeyboardShortcuts.ts

@@ -0,0 +1,90 @@
+import { useEffect, useCallback } from 'react'
+
+interface ShortcutConfig {
+  key: string
+  ctrl?: boolean
+  shift?: boolean
+  alt?: boolean
+  handler: () => void
+  enabled?: boolean
+}
+
+export function useKeyboardShortcuts(shortcuts: ShortcutConfig[]) {
+  const handleKeyDown = useCallback(
+    (e: KeyboardEvent) => {
+      // Don't trigger shortcuts when typing in inputs
+      const target = e.target as HTMLElement
+      if (
+        target.tagName === 'INPUT' ||
+        target.tagName === 'TEXTAREA' ||
+        target.isContentEditable
+      ) {
+        return
+      }
+
+      for (const shortcut of shortcuts) {
+        if (shortcut.enabled === false) continue
+
+        const keyMatch = e.key === shortcut.key || e.key === shortcut.key.toLowerCase()
+        const ctrlMatch = !!shortcut.ctrl === (e.ctrlKey || e.metaKey)
+        const shiftMatch = !!shortcut.shift === e.shiftKey
+        const altMatch = !!shortcut.alt === e.altKey
+
+        if (keyMatch && ctrlMatch && shiftMatch && altMatch) {
+          e.preventDefault()
+          shortcut.handler()
+          return
+        }
+      }
+    },
+    [shortcuts]
+  )
+
+  useEffect(() => {
+    document.addEventListener('keydown', handleKeyDown)
+    return () => document.removeEventListener('keydown', handleKeyDown)
+  }, [handleKeyDown])
+}
+
+// Convenience hook for tree navigation specifically
+export interface TreeNavigationShortcutsConfig {
+  onSelectOption: (index: number) => void
+  onGoBack: () => void
+  onContinue: () => void
+  optionCount: number
+  canGoBack: boolean
+  canContinue: boolean
+}
+
+export function useTreeNavigationShortcuts({
+  onSelectOption,
+  onGoBack,
+  onContinue,
+  optionCount,
+  canGoBack,
+  canContinue,
+}: TreeNavigationShortcutsConfig) {
+  const shortcuts: ShortcutConfig[] = [
+    // Number keys 1-9 for options
+    ...Array.from({ length: Math.min(optionCount, 9) }, (_, i) => ({
+      key: String(i + 1),
+      handler: () => onSelectOption(i),
+    })),
+    // Escape to go back
+    {
+      key: 'Escape',
+      handler: onGoBack,
+      enabled: canGoBack,
+    },
+    // Enter to continue (for action nodes)
+    {
+      key: 'Enter',
+      handler: onContinue,
+      enabled: canContinue,
+    },
+  ]
+
+  useKeyboardShortcuts(shortcuts)
+}
+
+export default useKeyboardShortcuts