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>

tests/test_apk_search.py

@@ -71,7 +71,7 @@ class SearchTests(SettingsTest):
                 result = search.search(timeout=.03)
                 self.assertLess(time.monotonic() - started, .3)
                 self.assertTrue(result['apps'])
-                self.assertEqual([s['status'] for s in result['sources']], ['ok', 'timed out', 'error'])
+                self.assertEqual([s['status'] for s in result['sources']], ['ok', 'loading', 'error'])
                 search.search('other', timeout=.03)
                 self.assertEqual(calls, [''])
         finally: