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/scan.go

@@ -140,16 +140,18 @@ func runScan(dirPath string, workers int, ffprobePath string, noSync bool) error
 		return enc.Encode(cache)
 	}
 
-	// Pre-extract subtitle sidecars (text subs → WebVTT in a hidden ".unarr" dir)
-	// so playback gets instant subtitles and huge remuxes never hit the on-demand
-	// timeout. Best-effort + Ctrl-C interruptible (the scan itself is already saved).
-	if cfg.Library.CacheSubtitles {
+	// Pre-extract sidecars (text subs → WebVTT, panel frames → JPEG) into a hidden
+	// ".unarr" dir so playback gets instant subtitles/thumbnails and huge remuxes
+	// never hit the on-demand timeout. Best-effort + Ctrl-C interruptible (the scan
+	// itself is already saved).
+	if cfg.Library.CacheSubtitles || cfg.Library.CacheThumbnails {
 		if ff, err := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath); err == nil {
-			fmt.Fprintf(os.Stderr, "  Pre-extracting subtitles to cache… (Ctrl-C to skip)\n")
+			fmt.Fprintf(os.Stderr, "  Pre-extracting subtitles + thumbnails to cache… (Ctrl-C to skip)\n")
 			library.PrewarmSidecars(ctx, cache, library.PrewarmOptions{
-				FFmpegPath:     ff,
-				CacheSubtitles: true,
-				Workers:        2,
+				FFmpegPath:      ff,
+				CacheSubtitles:  cfg.Library.CacheSubtitles,
+				CacheThumbnails: cfg.Library.CacheThumbnails,
+				Workers:         2,
 			})
 		}
 	}