feat: make hover thumbnails optional (--no-thumbnails)

Thumbnails stay on by default. --no-thumbnails skips building the preview sprite, so the server does no extra work and the rest of the player still works. The frontend already shows no preview when the sprite is unavailable.

stream_server.py

@@ -310,6 +310,9 @@ async def scrub_meta(v: int | None = None):
     loop) the first time it's asked for, then reuses it from memory."""
     from fastapi import Response
     import json as _json
+    # Thumbnails are on by default; --no-thumbnails turns the whole thing off.
+    if not getattr(app.state, "thumbnails", True):
+        return Response(content='{"available": false}', media_type="application/json")
     video_path = _scrub_video_path(v)
     if not video_path or not os.path.exists(video_path):
         return Response(content='{"available": false}', media_type="application/json")
@@ -757,6 +760,12 @@ if __name__ == "__main__":
         help="Adaptive-codec colour fidelity (lossless = bit-exact; lower = "
              "smaller stream via lossy temporal delta). Chars always exact."
     )
+    playback.add_argument(
+        "--no-thumbnails",
+        action="store_true", default=False,
+        help="Turn off the hover thumbnails on the seek bar (skips building the "
+             "preview sprite). The rest of the player still works."
+    )
 
     # ── Server ──
     srv = parser.add_argument_group('\033[33mServer\033[0m')
@@ -784,6 +793,7 @@ if __name__ == "__main__":
     app.state.loop          = args.loop
     app.state.tolerance     = {"lossless": 0, "high": 4, "balanced": 8, "low": 16}[args.quality]
     app.state.debug         = args.debug
+    app.state.thumbnails    = not args.no_thumbnails
     global_default_cols     = args.cols if args.cols is not None else (450 if args.pixel else 200)
     app.state.cols          = global_default_cols
     app.state.rows          = args.rows