feat: subtitles, delete-all, auto refresh, and menu cleanup

Subtitles: a language preference in Settings drives what is shown and
what is downloaded. yt-dlp saves WebVTT sidecars next to each download,
including YouTube's auto-generated track, and the player attaches them
as <track> elements so they work offline. Sidecars are removed with
their video, and by Delete all.

WebVTT rather than muxed subtitle streams because WebKit reads a <track>
reliably and largely ignores subtitle tracks inside an MP4.

Downloads in progress now appear under the downloaded-only filter, so a
download you just started does not vanish from the list you are watching
it in. That view also gains a Delete all, behind a confirmation naming
what goes.

The feed refreshes on launch and whenever the player closes, so the
Refresh button is only for staleness.

Player: Delete moved to the footer and shortened, Open on YouTube is now
an external-link icon.

Removes the Edit and Help menus. Cut/Copy/Paste move to the app menu,
without which their shortcuts would stop working in the search field.
The webview context menu is suppressed outside text fields — its Reload
and Back items act on a page the app does not present as one.

Settings now warns that 4K AV1 plays back with artefacts: the files
decode cleanly in ffmpeg, so it is the built-in decoder, not the
download.

src/api.ts

@@ -24,8 +24,14 @@ export const listFeed = (filter: FeedFilter) =>
 
 export const refreshFeeds = () => invoke<RefreshSummary>("refresh_feeds");
 
-export const downloadVideo = (videoId: string, quality: Quality) =>
-  invoke<void>("download_video", { videoId, quality });
+export const downloadVideo = (videoId: string, quality: Quality, subLangs: string) =>
+  invoke<void>("download_video", { videoId, quality, subLangs });
+
+export const deleteAllDownloads = () => invoke<number>("delete_all_downloads");
+
+/** WebVTT sidecars beside a downloaded video, as [language, path] pairs. */
+export const listSubtitles = (videoId: string) =>
+  invoke<Array<[string, string]>>("list_subtitles", { videoId });
 
 export const savePlayback = (videoId: string, position: number, duration: number) =>
   invoke<void>("save_playback", { videoId, position, duration });