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/database/app_database_test.dart

@@ -20,6 +20,7 @@ import 'package:plezy/utils/active_client_scope.dart';
 import 'package:plezy/utils/media_server_http_client.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
+import '../test_helpers/download_fixtures.dart';
 import '../test_helpers/prefs.dart';
 
 final class _EnsureOpenTrackingInterceptor extends QueryInterceptor {
@@ -81,8 +82,8 @@ class _AppDatabaseTestSuite {
         // v20 dropped the profile_id FK so virtual Plex Home profiles can
         // persist join rows without a parent `profiles` row. Profile deletion
         // instead cleans up join rows explicitly (via the teardown flow's
-        // removeAllProfileConnectionsAndCleanup) before deleting the profile,
-        // so the cascade isn't needed.
+        // ProfileConnectionCleanup.removeAllProfileConnections) before deleting
+        // the profile, so the cascade isn't needed.
         final now = DateTime.now().millisecondsSinceEpoch;
         await db
             .into(db.connections)