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>

CONTRIBUTING.md

@@ -298,19 +298,19 @@ Nginx (port 2026) ← Unified entry point
 ```bash
 # Backend tests
 cd backend
-uv run pytest
+make test
 
-# Frontend checks
+# Frontend tests
 cd frontend
-pnpm check
+make test
 ```
 
 ### PR Regression Checks
 
-Every pull request runs the backend regression workflow at [.github/workflows/backend-unit-tests.yml](.github/workflows/backend-unit-tests.yml), including:
+Every pull request triggers the following CI workflows:
 
-- `tests/test_provisioner_kubeconfig.py`
-- `tests/test_docker_sandbox_mode_detection.py`
+- **Backend unit tests** — [.github/workflows/backend-unit-tests.yml](.github/workflows/backend-unit-tests.yml)
+- **Frontend unit tests** — [.github/workflows/frontend-unit-tests.yml](.github/workflows/frontend-unit-tests.yml)
 
 ## Code Style