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/types.ts

@@ -128,6 +128,9 @@ export const BULK_LIMITS: Array<{ value: number; label: string }> = [
 export const DEFAULT_BULK_LIMIT = 25;
 
 /** Preferred subtitle language: shown when available, and downloaded. */
+/** On by default: "None" means no captions anywhere, which is rarely wanted. */
+export const DEFAULT_SUB_LANG = "en";
+
 export const SUB_LANGS: Array<{ value: string; label: string }> = [
   { value: "off", label: "None" },
   { value: "en", label: "English" },