fix: auth token handling, add tests

- Read bearer token from set-auth-token header
- Add mounted checks to prevent setState after dispose
- Add mocktail for testing
- Add widget tests for login, clients, events screens
- Add unit tests for auth provider, API client
- 110 tests passing

pubspec.yaml

@@ -43,6 +43,7 @@ dev_dependencies:
   build_runner: ^2.4.8
   freezed: ^2.4.6
   json_serializable: ^6.7.1
+  mocktail: ^1.0.3
 
 flutter:
   uses-material-design: true