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/services/library/library-factory.test.ts

@@ -0,0 +1,62 @@
+/**
+ * Component: Library Service Factory Tests
+ * Documentation: documentation/features/audiobookshelf-integration.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { clearLibraryServiceCache, getLibraryService } from '@/lib/services/library';
+
+const MockPlexService = vi.hoisted(() => class MockPlexService {});
+const MockAbsService = vi.hoisted(() => class MockAbsService {});
+
+const configServiceMock = vi.hoisted(() => ({
+  getBackendMode: vi.fn(),
+}));
+
+vi.mock('@/lib/services/config.service', () => ({
+  getConfigService: () => configServiceMock,
+}));
+
+vi.mock('@/lib/services/library/PlexLibraryService', () => ({
+  PlexLibraryService: MockPlexService,
+}));
+
+vi.mock('@/lib/services/library/AudiobookshelfLibraryService', () => ({
+  AudiobookshelfLibraryService: MockAbsService,
+}));
+
+describe('Library service factory', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    clearLibraryServiceCache();
+  });
+
+  it('returns Plex service when backend mode is plex', async () => {
+    configServiceMock.getBackendMode.mockResolvedValue('plex');
+
+    const service = await getLibraryService();
+
+    expect(service).toBeInstanceOf(MockPlexService);
+  });
+
+  it('returns cached service when mode is unchanged', async () => {
+    configServiceMock.getBackendMode.mockResolvedValue('plex');
+
+    const first = await getLibraryService();
+    const second = await getLibraryService();
+
+    expect(first).toBe(second);
+  });
+
+  it('switches to Audiobookshelf service when mode changes', async () => {
+    configServiceMock.getBackendMode
+      .mockResolvedValueOnce('plex')
+      .mockResolvedValueOnce('audiobookshelf');
+
+    const first = await getLibraryService();
+    const second = await getLibraryService();
+
+    expect(first).toBeInstanceOf(MockPlexService);
+    expect(second).toBeInstanceOf(MockAbsService);
+  });
+});