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>

CONTRIBUTING.md

@@ -300,9 +300,13 @@ Nginx (port 2026) ← Unified entry point
 cd backend
 make test
 
-# Frontend tests
+# Frontend unit tests
 cd frontend
 make test
+
+# Frontend E2E tests (requires Chromium; builds and auto-starts the Next.js production server)
+cd frontend
+make test-e2e
 ```
 
 ### PR Regression Checks
@@ -311,6 +315,7 @@ Every pull request triggers the following CI workflows:
 
 - **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)
+- **Frontend E2E tests** — [.github/workflows/e2e-tests.yml](.github/workflows/e2e-tests.yml) (triggered only when `frontend/` files change)
 
 ## Code Style