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

src/test-setup.ts

@@ -1,10 +1,3 @@
-import { getTestBed } from '@angular/core/testing';
-import {
-  BrowserDynamicTestingModule,
-  platformBrowserDynamicTesting,
-} from '@angular/platform-browser-dynamic/testing';
-
-getTestBed().initTestEnvironment(
-  BrowserDynamicTestingModule,
-  platformBrowserDynamicTesting(),
-);
+// Vitest test setup for frontend unit tests.
+// Angular TestBed initialization is not required for standalone unit tests.
+// Add shared mocks, global beforeEach hooks, or polyfills here as needed.