feat: initialize frontend unit test framework with Vitest

- Add vitest ^4.0.8, jsdom, @vitest/coverage-v8 to devDependencies
- Configure vitest.config.ts for jsdom environment with src/**/*.spec.ts pattern
- Simplify test-setup.ts (no TestBed needed for standalone unit tests)
- Add example spec file src/app/app.spec.ts (5 tests) covering App component logic
- Add npm scripts: test:unit, test:unit:watch, test:unit:coverage
- Verified: 5/5 frontend tests pass, 86/86 backend pytest tests still pass

vitest.config.ts

@@ -1,11 +1,11 @@
-/** @type {import('vite').UserConfig} */
+/** @type {import('vitest/config').ViteUserConfig} */
 export default {
   test: {
     globals: true,
     environment: 'jsdom',
     include: ['src/**/*.spec.ts'],
     setupFiles: ['src/test-setup.ts'],
-    reporters: ['default', 'html'],
+    reporters: ['default'],
     coverage: {
       enabled: true,
       reportsDirectory: 'coverage/frontend',