test(I2): vitest component smoke tests for high-risk UI

Adds @vue/test-utils + happy-dom and mounts CredentialCard, DownloadEventRow,
PostCard, ActiveDownloadsPanel, QueueStatusBar, asserting they render without
throwing and surface key content — catching the dead-binding / render-error
class (e.g. the last_verified_at regression, guarded explicitly). Vuetify
components are left unresolved (Vue renders unknown elements + slots), so no
Vuetify-plugin setup is needed; only RouterLink is stubbed. Per-file
happy-dom env via docblock keeps the existing node-env specs untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -26,6 +26,8 @@
     "vue-tsc": "^2.0.0",
     "vite-plugin-vuetify": "^2.0.0",
     "sass": "^1.71.0",
-    "vitest": "^2.1.0"
+    "vitest": "^2.1.0",
+    "@vue/test-utils": "^2.4.0",
+    "happy-dom": "^15.0.0"
   }
 }