Subtitle settings: global [subtitles] config + per-channel overrides

Adds full subtitle control (backend + desktop UI; web UI follows).

## Model
- New [subtitles] config section: enabled / auto_generated / embed /
  format / langs. Sensible defaults (subs on, auto on, no embed/convert,
  all langs).
- DownloadOptions gains per-channel Option overrides: subtitles_enabled,
  subtitles_auto, subtitles_embed, subtitle_format (subtitle_langs
  already existed). None = defer to global.

## Resolver
- Downloader gains subtitle_defaults (the global section) + a single
  apply_subtitle_flags() that merges global + per-channel and emits the
  yt-dlp flags (--write-subs / --write-auto-subs / --sub-langs /
  --convert-subs / --embed-subs). Disabled = emit nothing.
- Replaces the hardcoded --write-subs --write-auto-subs at the main
  download + repair builders. Moved the --sub-langs emission out of
  DownloadOptions::apply into the resolver so global + override merge in
  one place.

## Desktop UI
- Settings screen: "Subtitles (global defaults)" section.
- Channel-options dialog: tri-state (Default/On/Off) combos for each
  override + a convert-format field.

## Wiring
- subtitle_defaults seeded at Downloader construction (app + web) and
  refreshed on settings save (desktop).

4 resolver tests cover global defaults, disabled, global format/embed/
langs, and per-channel override-wins. 84 pass.

Web UI rows are the remaining piece (next commit).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/web.rs

@@ -2304,13 +2304,14 @@ async fn serve(config: Config, shutdown_rx: std::sync::mpsc::Receiver<()>) {
     let watched = db.get_watched().unwrap_or_default();
     let positions = db.get_positions().unwrap_or_default();
 
-    let downloader = Downloader::new(
+    let mut downloader = Downloader::new(
         channels_root.clone(),
         config.player.browser.clone(),
         config.backup.max_concurrent,
         config.backup.use_bundled_ytdlp,
         config.backup.use_pot_provider,
     );
+    downloader.subtitle_defaults = config.subtitles.clone();
     let music_root = downloader.music_root();
     let _ = std::fs::create_dir_all(&music_root);
     let transcode = AtomicBool::new(config.web.transcode);