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/CLAUDE.md

@@ -17,10 +17,11 @@ DeerFlow Frontend is a Next.js 16 web interface for an AI agent system. It commu
 | `pnpm check`     | Lint + type check (run before committing)         |
 | `pnpm lint`      | ESLint only                                       |
 | `pnpm lint:fix`  | ESLint with auto-fix                              |
+| `pnpm test`      | Run unit tests with Vitest                        |
 | `pnpm typecheck` | TypeScript type check (`tsc --noEmit`)            |
 | `pnpm start`     | Start production server                           |
 
-No test framework is configured.
+Unit tests live under `tests/unit/` and mirror the `src/` layout (e.g., `tests/unit/core/api/stream-mode.test.ts` tests `src/core/api/stream-mode.ts`). Powered by Vitest; import source modules via the `@/` path alias.
 
 ## Architecture