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/vite.config.ts

@@ -6,7 +6,10 @@ export default defineConfig(() => ({
   plugins: [react()],
   server: {
     proxy: {
-      "/api": "http://127.0.0.1:3001",
+      "/api": {
+        target: "http://127.0.0.1:3001",
+        timeout: 120000,
+      },
     },
   },
   build: {