Add backend unit test framework and modularize settings UI

Introduced a Vitest-based backend unit testing framework with supporting scripts, helpers, and GitHub Actions integration. Refactored the admin settings page to a modular architecture, splitting monolithic logic into feature-specific tabs and hooks for improved maintainability and testability. Updated documentation to reflect the new testing setup and settings architecture, and added new dependencies for testing utilities.

tests/api/admin-backend-mode.routes.test.ts

@@ -0,0 +1,62 @@
+/**
+ * Component: Admin Backend Mode API Route Tests
+ * Documentation: documentation/testing.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+let authRequest: any;
+
+const requireAuthMock = vi.hoisted(() => vi.fn());
+const requireAdminMock = vi.hoisted(() => vi.fn());
+const configServiceMock = vi.hoisted(() => ({
+  getBackendMode: vi.fn(),
+  setMany: vi.fn(),
+}));
+const clearLibraryServiceCacheMock = vi.hoisted(() => vi.fn());
+
+vi.mock('@/lib/middleware/auth', () => ({
+  requireAuth: requireAuthMock,
+  requireAdmin: requireAdminMock,
+}));
+
+vi.mock('@/lib/services/config.service', () => ({
+  ConfigurationService: class {
+    getBackendMode = configServiceMock.getBackendMode;
+    setMany = configServiceMock.setMany;
+  },
+}));
+
+vi.mock('@/lib/services/library', () => ({
+  clearLibraryServiceCache: clearLibraryServiceCacheMock,
+}));
+
+describe('Admin backend mode route', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    authRequest = { user: { id: 'admin-1', role: 'admin' } };
+    requireAuthMock.mockImplementation((_req: any, handler: any) => handler(authRequest));
+    requireAdminMock.mockImplementation((_req: any, handler: any) => handler());
+  });
+
+  it('returns backend mode', async () => {
+    configServiceMock.getBackendMode.mockResolvedValue('plex');
+
+    const { GET } = await import('@/app/api/admin/backend-mode/route');
+    const response = await GET({} as any);
+    const payload = await response.json();
+
+    expect(payload.backendMode).toBe('plex');
+  });
+
+  it('updates backend mode and clears cache', async () => {
+    const { PUT } = await import('@/app/api/admin/backend-mode/route');
+    const response = await PUT({ json: vi.fn().mockResolvedValue({ mode: 'audiobookshelf' }) } as any);
+    const payload = await response.json();
+
+    expect(payload.success).toBe(true);
+    expect(clearLibraryServiceCacheMock).toHaveBeenCalled();
+  });
+});
+
+