feat(frontend): set up Vitest frontend testing infrastructure with CI workflow (#2147)

* feat: set up Vitest frontend testing infrastructure with CI workflow

Migrate existing 4 frontend test files from Node.js native test runner
(node:test + node:assert/strict) to Vitest, reorganize test directory
structure under tests/unit/ mirroring src/ layout, and add a dedicated
CI workflow for frontend unit tests.

- Add vitest as devDependency, remove tsx
- Create vitest.config.ts with @/ path alias
- Migrate tests to Vitest API (test/expect/vi)
- Rename .mjs test files to .ts
- Move tests from src/ to tests/unit/ (mirrors src/ layout)
- Add frontend/Makefile `test` target
- Add .github/workflows/frontend-unit-tests.yml (parallel to backend)
- Update CONTRIBUTING.md, README.md, AGENTS.md, CLAUDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: fix the lint error

* style: fix the lint error

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/README.md

@@ -35,7 +35,7 @@ pnpm dev
 # The app will be available at http://localhost:3000
 ```
 
-### Build
+### Build & Test
 
 ```bash
 # Type check
@@ -50,6 +50,9 @@ pnpm format:write
 # Lint
 pnpm lint
 
+# Run unit tests
+pnpm test
+
 # Build for production
 pnpm build
 
@@ -82,6 +85,8 @@ NEXT_PUBLIC_LANGGRAPH_BASE_URL="http://localhost:2024"
 ## Project Structure
 
 ```
+tests/
+└── unit/                   # Unit tests (mirrors src/ layout)
 src/
 ├── app/                    # Next.js App Router pages
 │   ├── api/                # API routes
@@ -119,6 +124,7 @@ src/
 | `pnpm dev`          | Start development server with Turbopack |
 | `pnpm build`        | Build for production                    |
 | `pnpm start`        | Start production server                 |
+| `pnpm test`         | Run unit tests with Vitest              |
 | `pnpm format`       | Check formatting with Prettier          |
 | `pnpm format:write` | Apply formatting with Prettier          |
 | `pnpm lint`         | Run ESLint                              |