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

test/features/clients/clients_test.dart

@@ -103,9 +103,8 @@ void main() {
       expect(clients.isEmpty, isTrue);
     });
   });
-}
 
-group('Client Form Validation', () {
+  group('Client Form Validation', () {
   test('first name is required', () {
     final firstName = '';
     final isValid = firstName.isNotEmpty;
@@ -156,4 +155,5 @@ group('Client Form Validation', () {
       expect(phone.isNotEmpty, isTrue);
     }
   });
-});
+  });
+}