refactor(webui): rename Overview tab to Providers

Mechanical rename: OverviewView.vue -> ProvidersView.vue (file, component
import, tab label/value in App.vue), plus doc-comment/README references
across webui/ that named the view by its old tab label.

The backend GET /admin/api/overview endpoint, adminOverviewResponse type,
and the frontend AdminOverviewResponse/dashboard.overview field all keep
their original names unchanged (API surface) -- only the UI tab label and
the component/file that renders it changed. No behavior or layout change
to the view itself.

webui/src/stores/dashboard.ts

@@ -4,12 +4,14 @@ import { AdminApiError, adminFetch } from '@/lib/api'
 import { useAuthStore } from '@/stores/auth'
 import type { AdminOverviewResponse, AdminUsageResponse } from '@/types/api'
 
-/** How often the Overview/Usage views poll (matches the replaced vanilla-JS page). */
+/** How often the Providers/Usage views poll (matches the replaced vanilla-JS page). */
 const POLL_MS = 5000
 
 /**
  * useDashboardStore polls GET /admin/api/overview and GET /admin/api/usage
- * together every POLL_MS, feeding the Overview and Usage views. A single
+ * together every POLL_MS, feeding the Providers and Usage views (the store's
+ * own `overview` field keeps the backend's name — see ProvidersView.vue's
+ * doc comment for the UI-label/API-name split). A single
  * store (not two) because both routes are always fetched together — the
  * old vanilla-JS page's own Promise.all([overview, usage]) pattern.
  */