feat(test): WP-46 — Vitest coverage (report-only)

Enable coverage via the @angular/build:unit-test builder's first-class options: a
`coverage` configuration on the test target (v8 provider, text-summary/html/lcov,
excludes for specs/stories/contracts/generated client) + `npm run test:coverage`.
Report-only (no thresholds) — visibility first. CI's test step now runs test:coverage
(strict superset: fails on a failing test, never on coverage). Baseline ~71% stmts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -11,6 +11,7 @@
     "build": "ng build",
     "watch": "ng build --watch --configuration development",
     "test": "ng test",
+    "test:coverage": "ng test --configuration coverage",
     "storybook": "ng run atomic-design-poc:storybook",
     "build-storybook": "ng run atomic-design-poc:build-storybook",
     "test-storybook": "test-storybook",
@@ -50,6 +51,7 @@
     "@storybook/addon-onboarding": "^10.4.6",
     "@storybook/angular": "^10.4.6",
     "@storybook/test-runner": "^0.24.4",
+    "@vitest/coverage-v8": "^4.1.9",
     "angular-eslint": "^22.0.0",
     "axe-playwright": "^2.2.2",
     "concurrently": "^10.0.3",