refactor: extract shared mixins and helpers, drop dead abstractions

Introduces shared seams for paginated views, D-pad reorder, media control
routing, async singletons and the device method channel, then points the
open-coded copies at them.

Also removes unused models and duplicated provider/server plumbing, folds
the twice-implemented artifact store in the server, and factors the
repeated Flutter toolchain prologue in CI into a composite action.

lib/screens/settings/add_plex_account_screen.dart

@@ -86,12 +86,11 @@ class _AddPlexAccountScreenState extends State<AddPlexAccountScreen> with AsyncF
             // to the profile, remove it again so a cancelled attach doesn't
             // leave a global account behind.
             if (!registration.existedBefore) {
-              await removePlexAccountConnectionAndCleanup(
-                account: connection,
+              await ProfileConnectionCleanup(
                 profileConnections: pcRegistry,
                 connections: connRegistry,
                 storage: storage,
-              );
+              ).removePlexAccountConnection(connection);
             }
             if (mounted) Navigator.of(context).pop(false);
             return true;