fix(bulk-upload): i18n hardcoded strings in BulkDropZone and FileSwitcherStrip

- Add bulk_drop_desc, bulk_select_files, bulk_drop_zone_label, bulk_remove_file
  keys to de/en/es message files
- BulkDropZone: use m.bulk_drop_zone_label(), m.bulk_drop_desc(),
  m.bulk_select_files() — removes all hardcoded German
- FileSwitcherStrip: use m.bulk_remove_file() on × button; move aria-live
  from <ul> to a dedicated visually-hidden region above the strip (screen
  readers now announce changes without coupling the live region to the list)
- Spec: import FileEntry from component instead of re-declaring; use
  data-remove-id selector instead of hardcoded German aria-label

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/en.json

@@ -868,5 +868,9 @@
 	"bulk_file_error_chip_label": "Upload failed",
 	"bulk_upload_progress": "{done} of {total} uploaded",
 	"bulk_partial_success": "{created} created, {failed} failed",
-	"bulk_all_failed": "All uploads failed"
+	"bulk_all_failed": "All uploads failed",
+	"bulk_drop_desc": "A separate document is created for each file. The title is pre-filled from the filename — all other fields apply to all documents.",
+	"bulk_select_files": "Select files",
+	"bulk_drop_zone_label": "Drop files here",
+	"bulk_remove_file": "Remove"
 }