Bulk Dell warranty sync + add-warranty from asset pages

- POST /warranty/sync/dell: auto-detect Dell hardware by service tag (asset
  serial) and create warranties for the ones Dell recognizes, in batches of 100.
  Only looks up assets that lack a dated warranty, so re-runs are cheap and
  non-Dell serials are filtered out by Dell. DellProvider.bulk_lookup batches
  tags over the cached token.
- "Sync Dell" button on the Warranties page with a toast summary.
- WarrantyPanel "Add one / Add-manage" links deep-link to the warranties add
  modal pre-linked to that asset (?addfor=); WarrantiesList opens it prefilled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/api/index.js

@@ -970,6 +970,9 @@ export const warrantyApi = {
   refresh(id) {
     return api.post(`/warranty/${id}/refresh`)
   },
+  syncDell() {
+    return api.post('/warranty/sync/dell')
+  },
   report() {
     return api.get('/warranty/report')
   }