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/package.json

@@ -15,6 +15,7 @@
     "preview": "next build && next start",
     "start": "next start",
     "test": "vitest run",
+    "test:e2e": "playwright test",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -93,6 +94,7 @@
   },
   "devDependencies": {
     "@eslint/eslintrc": "^3.3.1",
+    "@playwright/test": "^1.59.1",
     "@tailwindcss/postcss": "^4.0.15",
     "@types/gsap": "^3.0.0",
     "@types/node": "^20.14.10",