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/common/ThemeToggle.tsx

@@ -0,0 +1,36 @@
+import { Sun, Moon, Monitor } from 'lucide-react'
+import { useThemeStore } from '@/store/themeStore'
+import { cn } from '@/lib/utils'
+
+export function ThemeToggle() {
+  const { theme, setTheme } = useThemeStore()
+
+  const options = [
+    { value: 'light' as const, icon: Sun, label: 'Light' },
+    { value: 'dark' as const, icon: Moon, label: 'Dark' },
+    { value: 'system' as const, icon: Monitor, label: 'System' },
+  ]
+
+  return (
+    <div className="flex items-center rounded-md border border-input bg-background p-1">
+      {options.map(({ value, icon: Icon, label }) => (
+        <button
+          key={value}
+          onClick={() => setTheme(value)}
+          className={cn(
+            'rounded p-1.5 transition-colors',
+            theme === value
+              ? 'bg-accent text-accent-foreground'
+              : 'text-muted-foreground hover:text-foreground'
+          )}
+          aria-label={`Switch to ${label} theme`}
+          aria-pressed={theme === value}
+        >
+          <Icon className="h-4 w-4" />
+        </button>
+      ))}
+    </div>
+  )
+}
+
+export default ThemeToggle