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/components/TopBar.tsx

@@ -21,6 +21,8 @@ interface Props {
   onView: (v: ViewMode) => void;
   sidebarHidden: boolean;
   onShowSidebar: () => void;
+  /** Present only when there is something to delete. */
+  onDeleteAll?: () => void;
   /** Matches the sidebar's inset so the two headers share a baseline. */
   titleBarInset: boolean;
 }
@@ -58,7 +60,7 @@ export default function TopBar({
   search, onSearch, downloadedOnly, onDownloadedOnly, hideShorts, onHideShorts,
   online, reachable, forcedOffline, onToggleForcedOffline,
   onRefresh, refreshing, refreshProgress, resultCount, view, onView,
-  sidebarHidden, onShowSidebar, titleBarInset,
+  sidebarHidden, onShowSidebar, onDeleteAll, titleBarInset,
 }: Props) {
   const pct = refreshProgress && refreshProgress.total > 0
     ? (refreshProgress.done / refreshProgress.total) * 100
@@ -105,6 +107,20 @@ export default function TopBar({
           Downloaded only
         </Toggle>
 
+        {downloadedOnly && onDeleteAll && (
+          <button
+            onClick={onDeleteAll}
+            title="Delete every download"
+            className={`inline-flex ${CONTROL_H} cursor-pointer items-center whitespace-nowrap
+                        rounded-lg border border-slate-300 px-2.5 text-[12px] font-medium
+                        text-slate-500 hover:border-red-500 hover:text-red-600
+                        dark:border-slate-700 dark:text-slate-400 dark:hover:border-red-500
+                        dark:hover:text-red-400`}
+          >
+            Delete all
+          </button>
+        )}
+
         <Toggle active={hideShorts} onClick={() => onHideShorts(!hideShorts)}
           title="Hide Shorts from the feed">
           Hide Shorts