video player: subtitle track support with a selector

Add /file-browser/subtitles (list text subtitle tracks) and /subtitle-vtt
(extract one as WebVTT on demand); the VideoRenderer fetches the list,
renders <track> elements, and shows a CC selector to switch/turn off subs.
Track labels prefer title, then handler_name (mp4), then a real language,
falling back to Track N — so untagged tracks aren't shown as "und".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/api/file-browser/router.ts

@@ -261,6 +261,68 @@ router.get('/raw', async (ctx) => {
   });
 });
 
+// List a video's text-based subtitle tracks (for the in-browser player's selector)
+const TEXT_SUBTITLE_CODECS = new Set(['subrip', 'srt', 'ass', 'ssa', 'mov_text', 'webvtt', 'text', 'subviewer', 'microdvd']);
+
+router.get('/subtitles', async (ctx) => {
+  const user = ctx.get('user');
+  const rootDir = getRootDir(user, ctx.req.query('root') ?? undefined);
+  const relPath = (ctx.req.query('path') || '').replace(/^\/+/, '');
+  if (!relPath) throw errors.BAD_REQUEST('path is required');
+  const absPath = resolveUserPath(rootDir, relPath);
+
+  const proc = Bun.spawn(
+    ['ffprobe', '-v', 'error', '-select_streams', 's', '-show_entries', 'stream=codec_name:stream_tags=language,title,handler_name', '-of', 'json', absPath],
+    { stdout: 'pipe', stderr: 'ignore' },
+  );
+  const out = await new Response(proc.stdout).text();
+  await proc.exited;
+
+  type ProbeStream = { codec_name?: string; tags?: { language?: string; title?: string; handler_name?: string } };
+  let streams: ProbeStream[] = [];
+  try {
+    streams = (JSON.parse(out).streams as ProbeStream[]) ?? [];
+  } catch {
+    streams = [];
+  }
+
+  // mkv stores the track name in `title`; mp4/mov stores it in `handler_name` (default names like
+  // "SubtitleHandler" are generic and ignored).
+  const trackName = (tags: ProbeStream['tags']) => {
+    const handler = tags?.handler_name ?? '';
+    return tags?.title || (handler && !/Handler$/.test(handler) ? handler : '');
+  };
+
+  // `id` is the subtitle-relative index among ALL subtitle streams (what `-map 0:s:id` expects),
+  // so it is assigned before filtering out image-based tracks that can't become WebVTT.
+  const tracks = streams
+    .map((s, id) => ({ id, s }))
+    .filter(({ s }) => TEXT_SUBTITLE_CODECS.has((s.codec_name ?? '').toLowerCase()))
+    .map(({ id, s }) => ({ id, codec: s.codec_name ?? '', lang: s.tags?.language ?? '', title: trackName(s.tags) }));
+
+  return ctx.json(tracks);
+});
+
+// Extract one subtitle track as WebVTT for a <track> element
+router.get('/subtitle-vtt', async (ctx) => {
+  const user = ctx.get('user');
+  const rootDir = getRootDir(user, ctx.req.query('root') ?? undefined);
+  const relPath = (ctx.req.query('path') || '').replace(/^\/+/, '');
+  if (!relPath) throw errors.BAD_REQUEST('path is required');
+  const track = parseInt(ctx.req.query('track') ?? '', 10);
+  if (!Number.isInteger(track) || track < 0) throw errors.BAD_REQUEST('valid track is required');
+  const absPath = resolveUserPath(rootDir, relPath);
+
+  const proc = Bun.spawn(['ffmpeg', '-v', 'error', '-i', absPath, '-map', `0:s:${track}`, '-f', 'webvtt', 'pipe:1'], {
+    stdout: 'pipe',
+    stderr: 'ignore',
+  });
+
+  return new Response(proc.stdout as ReadableStream, {
+    headers: { 'Content-Type': 'text/vtt; charset=utf-8' },
+  });
+});
+
 // Save a cached result (ocr/tts/transcriptions/audio) next to the original file
 const CACHE_PREFIXES = ['cache/ocr/', 'cache/tts/', 'cache/transcriptions/', 'cache/audio/'];