ci: enforce 80% backend coverage gate and add frontend coverage reporting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/vite.config.ts

@@ -33,6 +33,18 @@ export default defineConfig({
     environment: 'jsdom',
     setupFiles: './src/test/setup.ts',
     include: ['src/**/*.{test,spec}.{ts,tsx}'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json-summary', 'html'],
+      include: ['src/**/*.{ts,tsx}'],
+      exclude: [
+        'src/test/**',
+        'src/types/**',
+        'src/**/*.d.ts',
+        'src/instrument.ts',
+        'src/main.tsx',
+      ],
+    },
   },
   build: {
     sourcemap: 'hidden', // Generate source maps but don't expose them publicly