feat(stream): cache scan-time thumbnail frames to the .unarr sidecar

Pre-extract the file panel's sample frames (10/30/50/70/90% of runtime, w=320)
during the library scan and write-through any on-demand /thumbnail request into
the hidden ".unarr/<name>.t<sec>w<width>.jpg" sidecar. The /thumbnail handler
serves a fresh sidecar instantly, so the characteristics panel and seekbar
previews stop re-running ffmpeg per request.

- mediainfo.sidecar: ThumbnailCachePath, ReadCachedThumbnail, WriteCachedThumbnail,
  ExtractThumbnailJPEG (mirrors engine.buildThumbnailArgs).
- library.PrewarmSidecars: also enqueues the panel frame positions (kept in
  lockstep with the web's THUMB_FRACTIONS / THUMB_WIDTH) per item with a duration.
- thumbnailHandler: cache-read → hit; miss → extract → write-through.
- config: library.cache_thumbnails (default true) + both cache toggles exposed in
  the interactive 'unarr config' library menu.

Local only by design — frames are the user's own content, never uploaded.

internal/cmd/config_menu.go

@@ -322,6 +322,14 @@ func configLibrary(cfg *config.Config) error {
 				Title("Allow file deletion from web UI?").
 				Description("When enabled, the web library's Delete button can permanently remove files from disk.\nOnly activate this if you understand that deleted files cannot be recovered.").
 				Value(&cfg.Library.AllowDelete),
+			huh.NewConfirm().
+				Title("Cache subtitles during scan?").
+				Description("Extract embedded text subtitles to WebVTT once during the scan and store them\nbeside the media (hidden .unarr dir) so playback subtitles are instant — and huge\nremuxes don't time out extracting on demand. Local only; nothing is uploaded.").
+				Value(&cfg.Library.CacheSubtitles),
+			huh.NewConfirm().
+				Title("Cache thumbnails during scan?").
+				Description("Pre-extract a few preview frames per file (hidden .unarr dir) so the file panel\nand seekbar previews load instantly. Small optimized JPEGs; local only.").
+				Value(&cfg.Library.CacheThumbnails),
 		),
 	).Run()
 }