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/package.json

@@ -14,6 +14,7 @@
     "lint:fix": "eslint . --ext .ts,.tsx --fix",
     "preview": "next build && next start",
     "start": "next start",
+    "test": "vitest run",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -105,7 +106,8 @@
     "tailwindcss": "^4.0.15",
     "tw-animate-css": "^1.4.0",
     "typescript": "^5.8.2",
-    "typescript-eslint": "^8.27.0"
+    "typescript-eslint": "^8.27.0",
+    "vitest": "^4.1.4"
   },
   "ct3aMetadata": {
     "initVersion": "7.40.0"