test: add comprehensive test suite for components, composables, and pages

Add 16 new test files covering:
- Composables: useBackend, useMeta, useDataJson
- Type classes: QueryResult, ResumeContent
- UI components: BadgeList, BadgeListCard
- Navbar components: LanguageSwitcher, ThemeSwitcher
- App components: AppNavbar, AppFooter
- VocalSynth components: Projects, Tools
- Pages: contact, resume, [...slug]

Tests focus on pure logic, interfaces, and component rendering where
possible, avoiding complex mocking of Nuxt auto-imported composables.

Total: 174 tests across 17 test files (including existing useApi tests).

app/composables/useBackend.test.ts

@@ -0,0 +1,97 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { useBackend } from './useBackend';
+import type { MetaResponse } from '~/types/api/meta';
+import type { ContactResponse } from '~/types/api/contact';
+
+// Mock useApi
+const mockGet = vi.fn();
+const mockPost = vi.fn();
+
+vi.mock('./useApi', () => ({
+  useApi: vi.fn(() => ({
+    get: mockGet,
+    post: mockPost,
+  })),
+}));
+
+describe('useBackend', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  describe('getMeta', () => {
+    it('should call useApi.get with /meta endpoint', () => {
+      const mockResult = {
+        data: ref<MetaResponse | null>({ version: '1.0.0', name: 'Test' }),
+        error: ref(null),
+        loading: ref(false),
+        run: vi.fn(),
+      };
+      mockGet.mockReturnValue(mockResult);
+
+      const { getMeta } = useBackend();
+      const result = getMeta();
+
+      expect(mockGet).toHaveBeenCalledWith('/meta');
+      expect(result).toBe(mockResult);
+    });
+
+    it('should return UseApiResponse with correct structure', () => {
+      const mockResult = {
+        data: ref<MetaResponse | null>(null),
+        error: ref(null),
+        loading: ref(false),
+        run: vi.fn(),
+      };
+      mockGet.mockReturnValue(mockResult);
+
+      const { getMeta } = useBackend();
+      const result = getMeta();
+
+      expect(result).toHaveProperty('data');
+      expect(result).toHaveProperty('error');
+      expect(result).toHaveProperty('loading');
+      expect(result).toHaveProperty('run');
+    });
+  });
+
+  describe('postContact', () => {
+    it('should call useApi.post with /contact endpoint and immediate=false', () => {
+      const mockResult = {
+        data: ref<ContactResponse | null>(null),
+        error: ref(null),
+        loading: ref(false),
+        run: vi.fn(),
+      };
+      mockPost.mockReturnValue(mockResult);
+
+      const { postContact } = useBackend();
+      const result = postContact();
+
+      expect(mockPost).toHaveBeenCalledWith('/contact', undefined, false);
+      expect(result).toBe(mockResult);
+    });
+
+    it('should return UseApiResponse with correct structure', () => {
+      const mockResult = {
+        data: ref<ContactResponse | null>(null),
+        error: ref(null),
+        loading: ref(false),
+        run: vi.fn(),
+      };
+      mockPost.mockReturnValue(mockResult);
+
+      const { postContact } = useBackend();
+      const result = postContact();
+
+      expect(result).toHaveProperty('data');
+      expect(result).toHaveProperty('error');
+      expect(result).toHaveProperty('loading');
+      expect(result).toHaveProperty('run');
+    });
+  });
+});