Artwork sources: every optional source falls back, within limits

- Any failure of a source image or SteamGridDB (HTTPException, odd JSON) now
  becomes a warning and generated art, never an aborted install; refresh-art
  --all reports each app and carries on.
- URL artwork goes through apk_sources._images: public addresses only, at most
  three redirects, and one overall deadline for all of an install's fetches.
- PNGs are checked from their header only (any depth or interlace; Steam's
  Chromium decodes them), JPEGs may have trailing padding, and 4K screenshots
  are within limits. The slow pure-Python decoder is gone.
- SteamGridDB title matching keeps letters of every script and never matches
  on an empty name. One warning per source slot, not per candidate.

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

ui/frame_android.py

@@ -256,8 +256,8 @@ def refresh_art(pkg=None, artwork=None):
         for app in list_apps():
             try:
                 results.append(refresh_art(app['package'], artwork))
-            except (FrameError, OSError, ValueError) as e:
-                results.append({'package': app['package'], 'error': str(e)})
+            except Exception as e:  # one app's failure must not stop the others
+                results.append({'package': app['package'], 'label': app.get('label'), 'error': str(e) or type(e).__name__})
         return results
     with _install_lock:
         m = _meta_or_fail(pkg)