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

static/css/components/metadata-refresh-result.css

@@ -55,6 +55,10 @@
     border-left-color: var(--color-warning);
 }
 
+.stat-card-offline {
+    border-left-color: var(--text-secondary);
+}
+
 .stat-card-total {
     border-left-color: var(--lora-border);
 }