Merge the deduplication and dead-code removal pass

Consolidates duplicated logic behind shared implementations — paginated
grid tabs, focus chrome, cached remote stores, sheet selection columns,
the server artifact store and a test fixture layer — and removes code
that had become unreachable. Net reduction of about 5,500 lines with no
behaviour change.

Where a fix had landed separately in code that moved into a shared
helper, the fix was re-applied inside the helper rather than left behind
in the copy that went away.

test/startup_bootstrap_test.dart

@@ -13,6 +13,7 @@ import 'package:plezy/media/ids.dart';
 import 'package:plezy/models/download_models.dart';
 import 'package:plezy/services/base_shared_preferences_service.dart';
 
+import 'test_helpers/download_fixtures.dart';
 import 'test_helpers/prefs.dart';
 
 final class _OpenTrackingInterceptor extends QueryInterceptor {