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>
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