Store: real-source fixes found by running search against live repos

- F-Droid: percent-encode repo file names (a '#' in one screenshot name broke
  the whole main repo); a bad image name drops that image, not the app.
- Search: sources still fetching report 'loading' (UI says so and refreshes
  quietly); indexes warm up at server start; page-only SideQuest is not
  searched and appears as a 'Browse SideQuest' link instead of an error.
- Browse (empty query) ranks VR, artwork and recent updates first; the F-Droid
  archive is off by default (old versions only).
- Throttled sources fall back to their last cached copy; per-host message.
- Curated GitHub list gains Open Saber Plus (MIT) with icon and screenshots.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/server.py

@@ -1623,6 +1623,7 @@ def main():
     args = ap.parse_args()
     httpd = ThreadingHTTPServer(("127.0.0.1", args.port), Handler)
     sweep_tmp()
+    threading.Thread(target=apk_search.warm, daemon=True).start()  # big indexes download before the first search
     if not frame_host.WINDOWS:
         signal.signal(signal.SIGTERM, lambda *_: (_ for _ in ()).throw(KeyboardInterrupt))
     if args.exit_on_eof: