Add unit tests for OAuthButtons and LinkedAccountsSettings components

- Introduced comprehensive test coverage for `OAuthButtons` and `LinkedAccountsSettings`, including loading states, button behaviors, error handling, and custom class support.
- Implemented `LinkedAccountsPage` tests for rendering and component integration.
- Adjusted E2E coverage exclusions in various components, focusing on UI-heavy and animation-based flows best suited for E2E tests.
- Refined Jest coverage thresholds to align with improved unit test additions.

frontend/tests/components/auth/OAuthButtons.test.tsx

@@ -0,0 +1,216 @@
+/**
+ * Tests for OAuthButtons Component
+ */
+
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import { OAuthButtons } from '@/components/auth/OAuthButtons';
+import { useOAuthProviders, useOAuthStart } from '@/lib/api/hooks/useOAuth';
+
+// Mock the OAuth hooks
+jest.mock('@/lib/api/hooks/useOAuth', () => ({
+  useOAuthProviders: jest.fn(),
+  useOAuthStart: jest.fn(),
+}));
+
+// Mock next-intl
+jest.mock('next-intl', () => ({
+  useTranslations: () => (key: string, params?: { provider?: string }) => {
+    const translations: Record<string, string> = {
+      loading: 'Loading...',
+      divider: 'or continue with',
+      continueWith: `Continue with ${params?.provider || ''}`,
+      signUpWith: `Sign up with ${params?.provider || ''}`,
+    };
+    return translations[key] || key;
+  },
+}));
+
+// Mock config - must be complete to avoid undefined access
+jest.mock('@/config/app.config', () => ({
+  __esModule: true,
+  default: {
+    oauth: {
+      enabled: true,
+      providers: {
+        google: { name: 'Google', enabled: true },
+        github: { name: 'GitHub', enabled: true },
+      },
+      callbackPath: '/auth/callback',
+    },
+    routes: {
+      dashboard: '/dashboard',
+      login: '/login',
+      profile: '/settings/profile',
+    },
+    app: {
+      url: 'http://localhost:3000',
+    },
+  },
+}));
+
+describe('OAuthButtons', () => {
+  const mockProviders = {
+    enabled: true,
+    providers: [
+      { provider: 'google', name: 'Google' },
+      { provider: 'github', name: 'GitHub' },
+    ],
+  };
+
+  const mockOAuthStart = {
+    mutateAsync: jest.fn(),
+    isPending: false,
+  };
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    (useOAuthProviders as jest.Mock).mockReturnValue({
+      data: mockProviders,
+      isLoading: false,
+      error: null,
+    });
+    (useOAuthStart as jest.Mock).mockReturnValue(mockOAuthStart);
+  });
+
+  it('renders nothing when OAuth is disabled', () => {
+    (useOAuthProviders as jest.Mock).mockReturnValue({
+      data: { enabled: false, providers: [] },
+      isLoading: false,
+      error: null,
+    });
+
+    const { container } = render(<OAuthButtons mode="login" />);
+    expect(container).toBeEmptyDOMElement();
+  });
+
+  it('renders nothing when no providers available', () => {
+    (useOAuthProviders as jest.Mock).mockReturnValue({
+      data: { enabled: true, providers: [] },
+      isLoading: false,
+      error: null,
+    });
+
+    const { container } = render(<OAuthButtons mode="login" />);
+    expect(container).toBeEmptyDOMElement();
+  });
+
+  it('renders nothing when there is an error', () => {
+    (useOAuthProviders as jest.Mock).mockReturnValue({
+      data: mockProviders,
+      isLoading: false,
+      error: new Error('Failed to fetch'),
+    });
+
+    const { container } = render(<OAuthButtons mode="login" />);
+    expect(container).toBeEmptyDOMElement();
+  });
+
+  it('shows loading state', () => {
+    (useOAuthProviders as jest.Mock).mockReturnValue({
+      data: null,
+      isLoading: true,
+      error: null,
+    });
+
+    render(<OAuthButtons mode="login" />);
+
+    expect(screen.getByText('Loading...')).toBeInTheDocument();
+    expect(screen.getByRole('button')).toBeDisabled();
+  });
+
+  it('renders provider buttons in login mode', () => {
+    render(<OAuthButtons mode="login" />);
+
+    expect(screen.getByText('Continue with Google')).toBeInTheDocument();
+    expect(screen.getByText('Continue with GitHub')).toBeInTheDocument();
+  });
+
+  it('renders provider buttons in register mode', () => {
+    render(<OAuthButtons mode="register" />);
+
+    expect(screen.getByText('Sign up with Google')).toBeInTheDocument();
+    expect(screen.getByText('Sign up with GitHub')).toBeInTheDocument();
+  });
+
+  it('renders divider when showDivider is true (default)', () => {
+    render(<OAuthButtons mode="login" />);
+
+    expect(screen.getByText('or continue with')).toBeInTheDocument();
+  });
+
+  it('does not render divider when showDivider is false', () => {
+    render(<OAuthButtons mode="login" showDivider={false} />);
+
+    expect(screen.queryByText('or continue with')).not.toBeInTheDocument();
+  });
+
+  it('calls OAuth start when clicking provider button', async () => {
+    render(<OAuthButtons mode="login" />);
+
+    const googleButton = screen.getByText('Continue with Google');
+    fireEvent.click(googleButton);
+
+    await waitFor(() => {
+      expect(mockOAuthStart.mutateAsync).toHaveBeenCalledWith({
+        provider: 'google',
+        mode: 'login',
+      });
+    });
+  });
+
+  it('calls onStart callback when OAuth flow starts', async () => {
+    const onStart = jest.fn();
+    render(<OAuthButtons mode="login" onStart={onStart} />);
+
+    const googleButton = screen.getByText('Continue with Google');
+    fireEvent.click(googleButton);
+
+    await waitFor(() => {
+      expect(onStart).toHaveBeenCalledWith('google');
+    });
+  });
+
+  it('calls onError callback when OAuth start fails', async () => {
+    const error = new Error('OAuth failed');
+    mockOAuthStart.mutateAsync.mockRejectedValue(error);
+    const onError = jest.fn();
+
+    render(<OAuthButtons mode="login" onError={onError} />);
+
+    const googleButton = screen.getByText('Continue with Google');
+    fireEvent.click(googleButton);
+
+    await waitFor(() => {
+      expect(onError).toHaveBeenCalledWith(error);
+    });
+  });
+
+  it('disables buttons while OAuth is pending', () => {
+    (useOAuthStart as jest.Mock).mockReturnValue({
+      ...mockOAuthStart,
+      isPending: true,
+    });
+
+    render(<OAuthButtons mode="login" />);
+
+    const buttons = screen.getAllByRole('button');
+    buttons.forEach((button) => {
+      expect(button).toBeDisabled();
+    });
+  });
+
+  it('applies custom className', () => {
+    render(<OAuthButtons mode="login" className="custom-class" />);
+
+    const container = document.querySelector('.custom-class');
+    expect(container).toBeInTheDocument();
+  });
+
+  it('renders provider icons', () => {
+    render(<OAuthButtons mode="login" />);
+
+    // Icons are SVG elements
+    const svgs = document.querySelectorAll('svg');
+    expect(svgs.length).toBeGreaterThan(0);
+  });
+});