feat(app): sign in anonymously on first launch

The app's first patterns. Dependencies (lib/src/core/dependencies) is the composition root, built at startup and handed down by DependenciesScope; it replaces the scaffold's global client and restores a stored session before the first frame, so a restart never creates a second anonymous account. AuthGateway with ServerpodAuthGateway is the boundary to the server's authentication, AuthRepository signs in anonymously when nobody is signed in, and SessionBloc (flutter_bloc) keeps a person signed in, also after signing out or a revoked session. SessionGate shows a spinner while signing in, offers to try again when that fails, and builds the app anew for every person. AccountButton explains that progress is saved on this device and asks before signing out of an anonymous account, which can't be restored. AppBlocObserver logs all blocs.

Tests: the repository, the bloc's sequences (a stored session is kept, a failed sign-in is retried, signing out starts anew), and the widgets, including that cancelling the sign-out keeps the account. test/support holds the fake gateway and testDependencies(). AGENTS.md describes the composition root and the test factory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

pubspec.lock

@@ -104,6 +104,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.0.13"
+  bloc:
+    dependency: transitive
+    description:
+      name: bloc
+      sha256: e03b235924e4f509c27b5d6b2f949200e0a91149a9818b4f65eeb56662b75413
+      url: "https://pub.dev"
+    source: hosted
+    version: "9.2.1"
   boolean_selector:
     dependency: transitive
     description:
@@ -413,6 +421,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_bloc:
+    dependency: transitive
+    description:
+      name: flutter_bloc
+      sha256: cf51747952201a455a1c840f8171d273be009b932c75093020f9af64f2123e38
+      url: "https://pub.dev"
+    source: hosted
+    version: "9.1.1"
   flutter_driver:
     dependency: transitive
     description: flutter