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/Player.tsx

@@ -1,7 +1,9 @@
 import { useCallback, useEffect, useRef, useState } from "react";
-import { fileUrl, listSubtitles, openExternal, resolveStream, savePlayback } from "../api";
+import {
+  fetchSubtitles, fileUrl, listSubtitles, openExternal, resolveStream, savePlayback,
+} from "../api";
 import type { FeedItem } from "../types";
-import { compactViews, relativeTime } from "./format";
+import { compactViews, relativeTime, subtitleLabel } from "./format";
 import PlayerControls from "./PlayerControls";
 import { Badge, BTN, Spinner } from "./ui";
 
@@ -127,6 +129,41 @@ export default function Player({
       cancelled = true;
     };
   }, [item.id, path]);
+  // Captions fetched on demand, as (language, blob URL). YouTube's HLS
+  // manifest carries a dozen audio renditions and no subtitles at all, so a
+  // stream has nothing to show without this; a download saved before subtitles
+  // were switched on is in the same position. Blob URLs share the document's
+  // origin, which a file:// or 127.0.0.1 track would not.
+  const [fetched, setFetched] = useState<Array<[string, string]>>([]);
+  const [fetchingSubs, setFetchingSubs] = useState(false);
+
+  useEffect(() => {
+    setFetched([]);
+    if (subLang === "off" || sidecars.length > 0) return;
+    const urls: string[] = [];
+    let cancelled = false;
+    setFetchingSubs(true);
+    fetchSubtitles(item.id, subLang)
+      .then((list) => {
+        if (cancelled) return;
+        setFetched(
+          list.map(([lang, text]) => {
+            const url = URL.createObjectURL(new Blob([text], { type: "text/vtt" }));
+            urls.push(url);
+            return [lang, url] as [string, string];
+          }),
+        );
+      })
+      .catch(() => {
+        /* no captions in this language is an ordinary outcome */
+      })
+      .finally(() => !cancelled && setFetchingSubs(false));
+    return () => {
+      cancelled = true;
+      for (const u of urls) URL.revokeObjectURL(u);
+    };
+  }, [item.id, subLang, sidecars.length]);
+
   // Controls and edge arrows fade away while you are just watching.
   const [chromeVisible, setChromeVisible] = useState(true);
   const hideTimer = useRef<number | undefined>(undefined);
@@ -362,10 +399,19 @@ export default function Player({
                 key={file}
                 kind="subtitles"
                 srcLang={lang}
-                label={lang}
+                label={subtitleLabel(lang)}
                 src={fileUrl(file)}
               />
             ))}
+            {fetched.map(([lang, url]) => (
+              <track
+                key={url}
+                kind="subtitles"
+                srcLang={lang}
+                label={subtitleLabel(lang)}
+                src={url}
+              />
+            ))}
           </video>
         ) : (
           error && (
@@ -394,6 +440,7 @@ export default function Player({
               onActivity={showChrome}
               subLang={subLang}
               onSubLang={onSubLang}
+              subsLoading={fetchingSubs}
             />
           </div>
         )}