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-tauri/src/downloader.rs

@@ -79,7 +79,12 @@ pub fn parse_progress_line(line: &str) -> Option<Progress> {
 
 /// Arguments for downloading one video. Kept separate from process spawning so
 /// the argument construction is assertable in tests.
-pub fn build_args(video_id: &str, out_template: &str, quality: &str) -> Vec<String> {
+pub fn build_args(
+    video_id: &str,
+    out_template: &str,
+    quality: &str,
+    sub_langs: &str,
+) -> Vec<String> {
     vec![
         "-f".into(),
         format_selector(quality),
@@ -94,6 +99,18 @@ pub fn build_args(video_id: &str, out_template: &str, quality: &str) -> Vec<Stri
         "180".into(),
         "--progress-template".into(),
         PROGRESS_TEMPLATE.into(),
+        // Subtitles come along for offline use, including YouTube's
+        // auto-generated ones. They are written beside the video as WebVTT
+        // rather than muxed in: WebKit reads a <track> reliably, whereas
+        // subtitle streams inside an MP4 it largely ignores.
+        "--write-subs".into(),
+        "--write-auto-subs".into(),
+        "--sub-format".into(),
+        "vtt".into(),
+        "--convert-subs".into(),
+        "vtt".into(),
+        "--sub-langs".into(),
+        sub_langs.into(),
         "--print".into(),
         "after_move:FTPATH %(filepath)s".into(),
         "-o".into(),
@@ -175,9 +192,20 @@ mod tests {
         assert_eq!(p.pct(), None);
     }
 
+    #[test]
+    fn subtitles_are_requested_including_auto_generated() {
+        let args = build_args("abc", "/tmp/o.%(ext)s", "best", "en.*,nl.*");
+        assert!(args.contains(&"--write-subs".to_string()));
+        // The auto-generated track is the only one many videos have.
+        assert!(args.contains(&"--write-auto-subs".to_string()));
+        assert!(args.contains(&"en.*,nl.*".to_string()));
+        // WebVTT, because that is what a <track> element can load.
+        assert!(args.contains(&"vtt".to_string()));
+    }
+
     #[test]
     fn best_quality_takes_the_highest_available() {
-        let args = build_args("abc123", "/tmp/out.%(ext)s", "best");
+        let args = build_args("abc123", "/tmp/out.%(ext)s", "best", "en.*");
         assert!(args.contains(&FORMAT_BEST.to_string()));
         assert!(args.contains(&"mp4".to_string()));
         assert!(args.contains(&"https://www.youtube.com/watch?v=abc123".to_string()));
@@ -197,7 +225,7 @@ mod tests {
         let sel = format_selector("1080");
         assert!(sel.contains("height<=1080"));
         assert!(!sel.contains("height<=2160"));
-        assert!(build_args("x", "o", "1080").contains(&sel));
+        assert!(build_args("x", "o", "1080", "en.*").contains(&sel));
     }
 
     #[test]