feat(frontend): add Playwright E2E tests with CI workflow (#2279)

* feat(frontend): add Playwright E2E tests with CI workflow

Add end-to-end testing infrastructure using Playwright (Chromium only).
14 tests across 5 spec files cover landing page, chat workspace,
thread history, sidebar navigation, and agent chat — all with mocked
LangGraph/Backend APIs via network interception (zero backend dependency).

New files:
- playwright.config.ts — Chromium, 30s timeout, auto-start Next.js
- tests/e2e/utils/mock-api.ts — shared API mocks & SSE stream helpers
- tests/e2e/{landing,chat,thread-history,sidebar,agent-chat}.spec.ts
- .github/workflows/e2e-tests.yml — push main + PR trigger, paths filter

Updated: package.json, Makefile, .gitignore, CONTRIBUTING.md,
frontend/CLAUDE.md, frontend/AGENTS.md, frontend/README.md

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

* fix: apply Copilot suggestions

---------

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

frontend/AGENTS.md

@@ -37,6 +37,7 @@ DeerFlow is built on a sophisticated agent-based architecture using the [LangGra
 
 ```
 tests/
+├── e2e/                    # E2E tests (Playwright, Chromium, mocked backend)
 └── unit/                   # Unit tests (mirrors src/ layout, powered by Vitest)
 src/
 ├── app/                    # Next.js App Router pages
@@ -98,7 +99,7 @@ When adding new agent features:
 1. Follow the established project structure
 2. Add comprehensive TypeScript types
 3. Implement proper error handling
-4. Write unit tests under `tests/unit/` (run with `pnpm test`)
+4. Write unit tests under `tests/unit/` (run with `pnpm test`) and E2E tests under `tests/e2e/` (run with `pnpm test:e2e`)
 5. Update this documentation
 6. Follow the code style guide (ESLint + Prettier)