feat(frontend): add admin card to generate thumbnails with polling

Fourth card on /admin/system mirrors the mass-import pattern:
- POST /api/admin/generate-thumbnails to trigger
- 2000 ms polling on /api/admin/thumbnail-status while RUNNING
- processed / skipped / failed counters in the DONE message
- standalone pollInterval so import and thumbnail polling don't
  interfere with each other

Paraglide keys added in de/en/es, mirroring admin_system_import_*.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/en.json

@@ -338,6 +338,13 @@
 	"admin_system_import_status_running": "Import running…",
 	"admin_system_import_status_done": "Import complete – {count} documents processed.",
 	"admin_system_import_status_failed": "Error: {message}",
+	"admin_system_thumbnails_heading": "Generate thumbnails",
+	"admin_system_thumbnails_description": "Generates preview images for documents without a thumbnail (e.g. after the mass import).",
+	"admin_system_thumbnails_btn_start": "Generate thumbnails",
+	"admin_system_thumbnails_btn_retry": "Run again",
+	"admin_system_thumbnails_status_running": "Thumbnail generation running…",
+	"admin_system_thumbnails_status_done": "Done — {processed} generated, {skipped} skipped, {failed} failed.",
+	"admin_system_thumbnails_status_failed": "Error: {message}",
 	"comp_expandable_show_more": "Show more",
 	"comp_expandable_show_less": "Show less",
 	"error_comment_not_found": "The comment could not be found.",