Add Playwright E2E testing infrastructure

- Add playwright-web-flake to flake.nix for NixOS browser support
- Pin @playwright/test@1.56.1 to match nixpkgs version
- Create playwright.config.ts with Chromium-only, auto-start dev server
- Add e2e/smoke.spec.ts with initial smoke tests
- Add .mcp.json for Claude browser control via MCP
- Update .gitignore for playwright artifacts
- Remove E2E test skip from spec.md Known Limitations
- Update specs/testing.md to require three-tier testing approach

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -10,6 +10,9 @@
     "lint:fix": "biome check --write .",
     "test": "vitest",
     "test:run": "vitest run",
+    "test:e2e": "playwright test",
+    "test:e2e:ui": "playwright test --ui",
+    "test:e2e:headed": "playwright test --headed",
     "db:setup": "npx tsx scripts/setup-db.ts"
   },
   "dependencies": {
@@ -31,6 +34,7 @@
   },
   "devDependencies": {
     "@biomejs/biome": "2.3.11",
+    "@playwright/test": "1.56.1",
     "@tailwindcss/postcss": "^4",
     "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.9.1",