feat(upload): add drag-and-drop bulk upload zone to home page

Adds a compact, unobtrusive drop zone between the search card and the
document list. Only visible to users with WRITE_ALL permission.

- Drag-and-drop or click-to-select multiple files at once
- Client-side MIME type validation with per-file error messages
- POSTs to /api/documents/quick-upload; refreshes list via invalidateAll()
- Inline feedback: success count + per-file errors
- i18n keys added to de/en/es message files

Closes #66 (frontend part)

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

frontend/messages/es.json

@@ -265,5 +265,10 @@
 	"doc_panel_annotation_thread_title": "Anotación",
 	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}",
 	"pdf_annotations_show": "Mostrar anotaciones",
-	"pdf_annotations_hide": "Ocultar anotaciones"
+	"pdf_annotations_hide": "Ocultar anotaciones",
+	"upload_drop_hint": "Soltar archivos o hacer clic para seleccionar",
+	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
+	"upload_success": "{count} documento(s) creado(s)",
+	"upload_invalid_type": "{filename}: formato de archivo no admitido",
+	"upload_error": "Error al subir {filename}"
 }