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/README.md

@@ -53,6 +53,12 @@ pnpm lint
 # Run unit tests
 pnpm test
 
+# One-time setup: install Playwright Chromium browser
+pnpm exec playwright install chromium
+
+# Run E2E tests (builds and starts production server automatically)
+pnpm test:e2e
+
 # Build for production
 pnpm build
 
@@ -86,6 +92,7 @@ NEXT_PUBLIC_LANGGRAPH_BASE_URL="http://localhost:2024"
 
 ```
 tests/
+├── e2e/                    # E2E tests (Playwright, Chromium, mocked backend)
 └── unit/                   # Unit tests (mirrors src/ layout)
 src/
 ├── app/                    # Next.js App Router pages
@@ -125,6 +132,7 @@ src/
 | `pnpm build`        | Build for production                    |
 | `pnpm start`        | Start production server                 |
 | `pnpm test`         | Run unit tests with Vitest              |
+| `pnpm test:e2e`     | Run E2E tests with Playwright           |
 | `pnpm format`       | Check formatting with Prettier          |
 | `pnpm format:write` | Apply formatting with Prettier          |
 | `pnpm lint`         | Run ESLint                              |