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/config.routes.test.ts

@@ -0,0 +1,59 @@
+/**
+ * Component: Config API Route Tests
+ * Documentation: documentation/testing.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const configServiceMock = vi.hoisted(() => ({
+  setMany: vi.fn(),
+  getAll: vi.fn(),
+  getCategory: vi.fn(),
+}));
+
+vi.mock('@/lib/services/config.service', () => ({
+  getConfigService: () => configServiceMock,
+}));
+
+describe('Config API routes', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('returns full configuration', async () => {
+    configServiceMock.getAll.mockResolvedValue({ plex_url: 'http://plex' });
+    const { GET } = await import('@/app/api/config/route');
+
+    const response = await GET();
+    const payload = await response.json();
+
+    expect(payload.config.plex_url).toBe('http://plex');
+  });
+
+  it('updates configuration values', async () => {
+    const { PUT } = await import('@/app/api/config/route');
+    const response = await PUT({
+      json: vi.fn().mockResolvedValue({
+        updates: [{ key: 'plex_url', value: 'http://plex' }],
+      }),
+    } as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(200);
+    expect(payload.updated).toBe(1);
+    expect(configServiceMock.setMany).toHaveBeenCalled();
+  });
+
+  it('returns category configuration', async () => {
+    configServiceMock.getCategory.mockResolvedValue({ plex_url: 'http://plex' });
+    const { GET } = await import('@/app/api/config/[category]/route');
+
+    const response = await GET({} as any, { params: Promise.resolve({ category: 'plex' }) });
+    const payload = await response.json();
+
+    expect(payload.category).toBe('plex');
+    expect(payload.config.plex_url).toBe('http://plex');
+  });
+});
+
+