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/de.json

@@ -2130,6 +2130,7 @@
             "statSuccess": "Erfolgreich",
             "statFailed": "Fehlgeschlagen",
             "statSkipped": "Übersprungen",
+            "statOffline": "[TODO: Translate] Drive Offline",
             "statTotal": "Gesamt geprüft",
             "statDuration": "Dauer",
             "successMessage": "Alle {count} {type}s erfolgreich aktualisiert!",