Story 27: Coverage tracking (full-stack)

Add end-to-end coverage tracking: backend collects vitest coverage,
records metrics with threshold/baseline tracking, and blocks acceptance
on regression. Frontend displays coverage in gate/review panels with
a "Collect Coverage" button. Includes 20 Rust tests, 17 Vitest tests,
and 14 Playwright E2E tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -10,7 +10,8 @@
     "server": "cargo run --manifest-path server/Cargo.toml",
     "test": "vitest run",
     "test:unit": "vitest run",
-    "test:e2e": "playwright test"
+    "test:e2e": "playwright test",
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "@types/react-syntax-highlighter": "^15.5.13",
@@ -25,10 +26,11 @@
     "@testing-library/jest-dom": "^6.0.0",
     "@testing-library/react": "^14.0.0",
     "@testing-library/user-event": "^14.4.3",
+    "@types/node": "^25.0.0",
     "@types/react": "^19.1.8",
     "@types/react-dom": "^19.1.6",
-    "@types/node": "^25.0.0",
     "@vitejs/plugin-react": "^4.6.0",
+    "@vitest/coverage-v8": "^2.1.9",
     "jest": "^29.0.0",
     "jsdom": "^28.1.0",
     "ts-jest": "^29.0.0",