feat(metadata): skip models on offline roots during bulk fetch (#1108)

Bulk metadata refresh picked candidates purely from cache state, so the
entries the scoped scan deliberately keeps for unreachable drives were
fetched anyway — burning CivitAI rate limit on sidecar/preview writes
that cannot land.

- Filter the fetch queue by root reachability (live check via
  describe_model_roots), covering both startup-offline and mid-session
  drive-off; probing failures degrade to no filtering
- Report offline_skipped / offline_roots in the WS terminal payloads,
  the HTTP result and the completion message
- Show a "Drive Offline" stat card in the fetch summary modal when
  any model was skipped this way

locales/en.json

@@ -2130,6 +2130,7 @@
       "statSuccess": "Success",
       "statFailed": "Failed",
       "statSkipped": "Skipped",
+      "statOffline": "Drive Offline",
       "statTotal": "Total Scanned",
       "statDuration": "Duration",
       "successMessage": "All {count} {type}s updated successfully!",