feat(apps): add core API infrastructure

apps/test/core/api/api_exception_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:social_app/core/api/api_exception.dart';
+
+void main() {
+  group('ApiException', () {
+    test('creates from DioException with 400 status', () {
+      final dioException = Exception('Bad request');
+      final apiException = ApiException.fromDioError(dioException);
+
+      expect(apiException, isA<ApiException>());
+      expect(apiException.message, contains('Request failed'));
+    });
+
+    test('NetworkException has correct message', () {
+      const exception = NetworkException('No internet');
+      expect(exception.message, 'No internet');
+    });
+
+    test('UnauthorizedException has default message', () {
+      const exception = UnauthorizedException();
+      expect(exception.message, 'Authentication required');
+    });
+  });
+}

apps/test/core/storage/token_storage_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:social_app/core/storage/token_storage.dart';
+
+class MockTokenStorage extends Mock implements TokenStorage {}
+
+void main() {
+  late TokenStorage storage;
+
+  setUp(() {
+    storage = MockTokenStorage();
+  });
+
+  group('TokenStorage', () {
+    test('saves and retrieves access token', () async {
+      when(
+        () => storage.getAccessToken(),
+      ).thenAnswer((_) async => 'test_access');
+      when(
+        () =>
+            storage.saveTokens(access: 'test_access', refresh: 'test_refresh'),
+      ).thenAnswer((_) async {});
+
+      await storage.saveTokens(access: 'test_access', refresh: 'test_refresh');
+      final token = await storage.getAccessToken();
+
+      expect(token, 'test_access');
+      verify(
+        () =>
+            storage.saveTokens(access: 'test_access', refresh: 'test_refresh'),
+      ).called(1);
+    });
+
+    test('clear removes all tokens', () async {
+      when(() => storage.clear()).thenAnswer((_) async {});
+      when(() => storage.getAccessToken()).thenAnswer((_) async => null);
+
+      await storage.clear();
+      final token = await storage.getAccessToken();
+
+      expect(token, isNull);
+    });
+  });
+}