Wire the existing `export_video` backend (#112) to a front-end surface so
users can render the timeline to a real .mp4 from the UI.

- ExportDialog: format select (H.264/.mp4 enabled; H.265/ProRes shown
  disabled + "not wired" note, matching the backend's resolve_preset
  rejection) and resolution select (720p/1080p/4K, default seeded from the
  timeline's own short edge). Export → native .mp4 save dialog → exportVideo.
  Backend has no progress callback, so this is a status+toast surface
  (button reads "Exporting…", controls disabled in flight); success/failure
  both pushToast.
- TitleBar: "Export Video" entry beside the XML export, disabled when no
  track holds a clip.
- api.ts: exportVideo() wrapper + ExportRequest/ExportSummary types,
  camelCase-aligned with the Rust DTO; rejects outside Tauri.
- Dropdown: optional per-option `disabled` (greyed, unselectable).
- uiStore: exportDialogOpen state; mount dialog in App.
- i18n: zh-CN + en keys for the dialog and title-bar entry.
- Tests: pure helpers (withMp4Ext / defaultMp4Name / defaultQuality).

Front-end only; no export backend logic changed.