chore: add root gradle coverage and frontendCoverage tasks

- frontendCoverage: runs 'npm run test:coverage' in frontend directory
  (vitest with coverage, enforces thresholds internally)
- coverage: group='verification', runs backend jacocoTestReport and
  frontendCoverage sequentially — single command for both layers:
  ./gradlew coverage
- check task continues to run only: frontendLint → frontendTest
  (coverage verification is added per-module: jacocoTestCoverage
  Verification on backend, vitest thresholds on frontend)

build.gradle

@@ -15,6 +15,18 @@ tasks.register('frontendTest', Exec) {
     commandLine 'npm', 'run', 'test'
 }
 
+tasks.register('frontendCoverage', Exec) {
+    description = 'Run Vitest with coverage in the frontend directory'
+    workingDir = file("${rootProject.projectDir}/frontend")
+    commandLine 'npm', 'run', 'test:coverage'
+}
+
+tasks.register('coverage') {
+    group = 'verification'
+    description = 'Run backend + frontend tests with coverage thresholds'
+    dependsOn(':backend:jacocoTestReport', 'frontendCoverage')
+}
+
 tasks.register('frontendE2E', Exec) {
     description = 'Run Playwright E2E tests in Docker (CI mode)'
     workingDir = file("${rootProject.projectDir}/frontend")