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>

angular.json

@@ -91,7 +91,22 @@
           "defaultConfiguration": "development"
         },
         "test": {
-          "builder": "@angular/build:unit-test"
+          "builder": "@angular/build:unit-test",
+          "configurations": {
+            "coverage": {
+              "coverage": true,
+              "coverageReporters": ["text-summary", "html", "lcov"],
+              "coverageExclude": [
+                "**/*.spec.ts",
+                "**/*.stories.ts",
+                "**/contracts/**",
+                "src/app/shared/infrastructure/api-client.ts",
+                "src/main.ts",
+                "src/test-setup.ts",
+                "**/*.d.ts"
+              ]
+            }
+          }
         },
         "storybook": {
           "builder": "@storybook/angular:start-storybook",