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/db.rs

@@ -369,7 +369,9 @@ impl Db {
             args.push(Box::new(pat));
         }
         if f.downloaded_only {
-            sql.push_str(" AND d.state = 'done'");
+            // Queued and running count: a download you started should not
+            // vanish from the very list you are watching it in.
+            sql.push_str(" AND d.state IN ('done','queued','running')");
         }
         if f.hide_shorts {
             sql.push_str(" AND v.is_short = 0");
@@ -498,6 +500,26 @@ impl Db {
             .map_err(|e| e.to_string())
     }
 
+    /// Paths of every completed download, for deleting them all at once.
+    pub fn all_download_paths(&self) -> Result<Vec<String>, String> {
+        let mut stmt = self
+            .conn
+            .prepare("SELECT path FROM downloads WHERE path IS NOT NULL")
+            .map_err(|e| e.to_string())?;
+        let rows = stmt
+            .query_map([], |r| r.get::<_, String>(0))
+            .map_err(|e| e.to_string())?;
+        rows.collect::<Result<Vec<_>, _>>().map_err(|e| e.to_string())
+    }
+
+    pub fn clear_all_downloads(&self) -> Result<usize, String> {
+        let n = self
+            .conn
+            .execute("DELETE FROM downloads", [])
+            .map_err(|e| e.to_string())?;
+        Ok(n)
+    }
+
     pub fn clear_download(&self, video_id: &str) -> Result<(), String> {
         self.conn
             .execute("DELETE FROM downloads WHERE video_id = ?1", params![video_id])
@@ -737,6 +759,36 @@ mod tests {
         assert_eq!(feed[0].id, "b");
     }
 
+    #[test]
+    fn downloads_in_progress_still_show_in_the_downloaded_filter() {
+        let db = seeded();
+        db.set_download_state("a", DownloadState::Running, None).unwrap();
+        db.set_download_state("c", DownloadState::Queued, None).unwrap();
+        db.set_download_state("b", DownloadState::Failed, Some("x")).unwrap();
+
+        let feed = db
+            .list_feed(&FeedFilter { downloaded_only: true, ..Default::default() })
+            .unwrap();
+        let ids: Vec<&str> = feed.iter().map(|f| f.id.as_str()).collect();
+        assert!(ids.contains(&"a"), "running should be listed");
+        assert!(ids.contains(&"c"), "queued should be listed");
+        assert!(!ids.contains(&"b"), "failed should not be");
+    }
+
+    #[test]
+    fn clearing_all_downloads_empties_the_filter() {
+        let db = seeded();
+        db.set_download_state("a", DownloadState::Done, None).unwrap();
+        db.set_download_path("a", "/movies/a.mp4").unwrap();
+        db.set_download_state("b", DownloadState::Done, None).unwrap();
+        assert_eq!(db.all_download_paths().unwrap(), vec!["/movies/a.mp4".to_string()]);
+        db.clear_all_downloads().unwrap();
+        assert!(db
+            .list_feed(&FeedFilter { downloaded_only: true, ..Default::default() })
+            .unwrap()
+            .is_empty());
+    }
+
     #[test]
     fn hide_shorts_filter_excludes_shorts() {
         let db = seeded();