chore(ui): add CodeMirror 6 + Playwright config

Install @codemirror/{view,state,autocomplete,commands,language,lint}
and @lezer/common — needed by Phase 3's MustacheEditor (Task 13).
CM6 picked over a raw textarea for its small incremental-rendering
bundle, full ARIA/keyboard support, and pluggable autocomplete +
linter APIs that map cleanly to Mustache token parsing.

Add ui/playwright.config.ts wiring Task 30's E2E smoke:
- testDir ./src/test/e2e, single worker, trace+screenshot on failure
- webServer launches `npm run dev:local` (backend on :8081 required)
- PLAYWRIGHT_BASE_URL env var skips the dev server for CI against a
  pre-deployed UI

Add test:e2e / test:e2e:ui npm scripts and exclude Playwright's
test-results/ and playwright-report/ from git. @playwright/test
itself was already in devDependencies from an earlier task.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/package.json

@@ -15,10 +15,19 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "test:ui": "vitest --ui",
+    "test:e2e": "playwright test",
+    "test:e2e:ui": "playwright test --ui",
     "postinstall": "node -e \"const fs=require('fs');fs.mkdirSync('public',{recursive:true});fs.copyFileSync('node_modules/@cameleer/design-system/assets/cameleer-logo.svg','public/favicon.svg')\""
   },
   "dependencies": {
     "@cameleer/design-system": "^0.1.56",
+    "@codemirror/autocomplete": "^6.20.1",
+    "@codemirror/commands": "^6.10.3",
+    "@codemirror/language": "^6.12.3",
+    "@codemirror/lint": "^6.9.5",
+    "@codemirror/state": "^6.6.0",
+    "@codemirror/view": "^6.41.1",
+    "@lezer/common": "^1.5.2",
     "@tanstack/react-query": "^5.90.21",
     "js-yaml": "^4.1.1",
     "lucide-react": "^1.7.0",