Library artwork backfill: devkit titles in refresh-art; missing art offered and re-applied

- 'Refresh artwork' (settings) and the API's refresh-art --all cover devkit
  titles as well as Android apps; frame_titles.py gains refresh-art ID|--all.
- Apps and titles without complete Steam artwork are flagged (art_missing):
  the app shows 'Add artwork', and the CLIs' list prints the refresh command.
- When the app lists them and Steam answers, Frame Control re-applies their
  art in the background (at most every five minutes), e.g. for a title Steam
  registered after an install made while it wasn't running.

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

ui/frame_android.py

@@ -377,6 +377,7 @@ def list_apps():
             continue
         if m:
             m['running'] = f"lepton-steamlaunch-{m['instance']}" in running
+            m['art_missing'] = art_missing(m)
             apps.append(m)
     return sorted(apps, key=lambda m: m['label'].lower())
 
@@ -537,6 +538,8 @@ def main():
             r = (backup_data if cmd == 'backup-data' else restore_data)(*args)
         elif cmd == 'list':
             r = list_apps()
+            if any(a['art_missing'] for a in r):
+                print('Some apps have no Steam artwork: python3 ui/frame_android.py refresh-art --all', file=sys.stderr)
         elif cmd in ('launch', 'stop', 'probe'):
             r = globals()[cmd](args[0])
         elif cmd == 'remove':