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/cache.routes.test.ts

@@ -0,0 +1,53 @@
+/**
+ * Component: Cache API Route Tests
+ * Documentation: documentation/testing.md
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const fsMock = vi.hoisted(() => ({
+  access: vi.fn(),
+  readFile: vi.fn(),
+}));
+
+vi.mock('fs/promises', () => ({ default: fsMock, ...fsMock, constants: { R_OK: 4 } }));
+
+describe('Thumbnail cache route', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('rejects invalid filenames', async () => {
+    const { GET } = await import('@/app/api/cache/thumbnails/[filename]/route');
+
+    const response = await GET({} as any, { params: Promise.resolve({ filename: '../bad' }) });
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toBe('Invalid filename');
+  });
+
+  it('returns 404 when file is missing', async () => {
+    fsMock.access.mockRejectedValueOnce(new Error('missing'));
+    const { GET } = await import('@/app/api/cache/thumbnails/[filename]/route');
+
+    const response = await GET({} as any, { params: Promise.resolve({ filename: 'file.jpg' }) });
+    const payload = await response.json();
+
+    expect(response.status).toBe(404);
+    expect(payload.error).toBe('File not found');
+  });
+
+  it('serves cached image with content type', async () => {
+    fsMock.access.mockResolvedValueOnce(undefined);
+    fsMock.readFile.mockResolvedValueOnce(Buffer.from('data'));
+    const { GET } = await import('@/app/api/cache/thumbnails/[filename]/route');
+
+    const response = await GET({} as any, { params: Promise.resolve({ filename: 'file.png' }) });
+
+    expect(response.status).toBe(200);
+    expect(response.headers.get('content-type')).toBe('image/png');
+  });
+});
+
+