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/vitest.config.ts

@@ -9,5 +9,10 @@ export default defineConfig({
     setupFiles: ["./src/setupTests.ts"],
     css: true,
     exclude: ["tests/e2e/**", "node_modules/**"],
+    coverage: {
+      provider: "v8",
+      reporter: ["json-summary"],
+      reportsDirectory: "./coverage",
+    },
   },
 });