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/playwright.config.ts

@@ -0,0 +1,26 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './src/test/e2e',
+  fullyParallel: false,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 1 : 0,
+  workers: 1,
+  reporter: process.env.CI ? [['html'], ['github']] : [['list']],
+  use: {
+    baseURL: process.env.PLAYWRIGHT_BASE_URL ?? 'http://localhost:5173',
+    trace: 'retain-on-failure',
+    screenshot: 'only-on-failure',
+  },
+  projects: [
+    { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
+  ],
+  webServer: process.env.PLAYWRIGHT_BASE_URL
+    ? undefined
+    : {
+        command: 'npm run dev:local',
+        url: 'http://localhost:5173',
+        reuseExistingServer: !process.env.CI,
+        timeout: 60_000,
+      },
+});