Add Vitest coverage for frontend tests

- Install @vitest/coverage-v8
- Configure coverage in vitest.config.ts
- Add npm script: test:coverage
- Add coverage/ to gitignore

frontend/vitest.config.ts

@@ -6,5 +6,10 @@ export default defineConfig({
   test: {
     environment: "jsdom",
     include: ["app/**/*.test.{ts,tsx}"],
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "html"],
+      exclude: ["app/generated/**", "**/*.test.{ts,tsx}", "e2e/**"],
+    },
   },
 });