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/components/ModelChip.vue

@@ -7,7 +7,7 @@ import { copyText } from '@/lib/clipboard'
 
 /**
  * ModelChip is one clickable, copyable model-id chip in a provider's
- * expanded model list (OverviewView.vue's provider-model accordion). id
+ * expanded model list (ProvidersView.vue's provider-model accordion). id
  * is the full routable id (lib/format.ts's routableModelId) — what
  * actually gets copied, so a pasted value works straight into a
  * `model: "..."` request field.