feat: subtitles while streaming, and fetched when a download lacks them

YouTube's HLS manifest carries a dozen audio renditions and no
subtitles whatsoever — checked against a live master playlist: 16
EXT-X-MEDIA entries, all TYPE=AUDIO, zero SUBTITLES. So the track menu
could only ever list audio while streaming, which is what it did.

Subtitles are now fetched separately with yt-dlp, tidied through the
existing VTT cleanup, and cached per video and language. They reach the
player as blob URLs, which share the document's origin — a file:// or
127.0.0.1 track would be cross-origin to the page and need CORS the
media pipeline cannot supply. The same path fills in a download saved
before subtitles were switched on, without fetching the video again.

YouTube serves identical auto-generated captions under both "en" and
"en-orig", so byte-identical texts collapse to one entry rather than
offering the same track twice, and tracks are labelled "English"
rather than "en".

The subtitle preference now defaults to English. "None" is a poor
default for a setting whose whole purpose is captions: it silently
means no subtitles are downloaded, fetched, or offered anywhere, and
the Settings text now says so.

src/components/Settings.tsx

@@ -276,9 +276,12 @@ export default function Settings({
                 </select>
               </label>
               <p className={`mt-2 ${HELP}`}>
-                Shown automatically when a video has subtitles in this language, including
-                YouTube's auto-generated ones, and saved alongside anything you download so
-                they work offline. You can still switch tracks from the player.
+                Shown whenever a video has subtitles in this language, including YouTube's
+                auto-generated ones — on most videos those are the only ones there are.
+                Downloads keep them beside the file for offline use; streams fetch them
+                separately, because YouTube's live manifest carries no subtitles at all.
+                <b> None means no subtitles anywhere.</b> You can still switch tracks from
+                the player.
               </p>
             </section>