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/App.tsx

@@ -16,7 +16,8 @@ import { useDownloads } from "./hooks/useDownloads";
 import { useFeed } from "./hooks/useFeed";
 import { useWindowFullscreen } from "./hooks/useWindowFullscreen";
 import {
-  BULK_LIMITS, DEFAULT_BULK_LIMIT, QUALITIES, STREAM_QUALITIES, SUB_LANGS,
+  BULK_LIMITS, DEFAULT_BULK_LIMIT, DEFAULT_SUB_LANG,
+  QUALITIES, STREAM_QUALITIES, SUB_LANGS,
   type FeedFilter, type FeedItem, type Quality, type RefreshProgress,
 } from "./types";
 
@@ -66,9 +67,9 @@ export default function App() {
   const [subLang, setSubLang] = useState(() => {
     try {
       const stored = localStorage.getItem("flighttube.subLang");
-      return SUB_LANGS.some((l) => l.value === stored) ? stored! : "off";
+      return SUB_LANGS.some((l) => l.value === stored) ? stored! : DEFAULT_SUB_LANG;
     } catch {
-      return "off";
+      return DEFAULT_SUB_LANG;
     }
   });
   const [streamQuality, setStreamQuality] = useState<Quality>(() => {