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/test/support/mountComponent.js

@@ -0,0 +1,24 @@
+import { mount } from '@vue/test-utils'
+import { createPinia, setActivePinia } from 'pinia'
+
+// Smoke-test helper. Vuetify components are left unresolved (Vue 3 renders
+// unknown elements with their slot children, so text content is still
+// present and nothing throws), which avoids standing up the whole Vuetify
+// plugin in happy-dom. RouterLink is the one import that needs a router, so
+// it's stubbed with a slot-rendering anchor.
+
+export function freshPinia () {
+  const pinia = createPinia()
+  setActivePinia(pinia)
+  return pinia
+}
+
+export function mountComponent (Component, { props = {}, pinia } = {}) {
+  return mount(Component, {
+    props,
+    global: {
+      plugins: pinia ? [pinia] : [],
+      stubs: { RouterLink: { template: '<a><slot /></a>' } },
+    },
+  })
+}