fix(bulk-upload): i18n topbar title; replace hardcoded German strings

'Neues Dokument' / 'Neue Dokumente' in BulkDocumentEditLayout topbar
bypassed Paraglide. Added bulk_title_single and bulk_title_multi keys
to de/en/es message files and switched to m.*() calls.

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

frontend/messages/de.json

@@ -872,5 +872,7 @@
 	"bulk_drop_desc": "Für jede Datei wird ein eigenes Dokument erstellt. Der Titel wird aus dem Dateinamen vorausgefüllt — alle anderen Felder gelten für alle gemeinsam.",
 	"bulk_select_files": "Dateien auswählen",
 	"bulk_drop_zone_label": "Dateien ablegen",
-	"bulk_remove_file": "Entfernen"
+	"bulk_remove_file": "Entfernen",
+	"bulk_title_single": "Neues Dokument",
+	"bulk_title_multi": "Neue Dokumente"
 }

frontend/messages/en.json

@@ -872,5 +872,7 @@
 	"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"
+	"bulk_remove_file": "Remove",
+	"bulk_title_single": "New Document",
+	"bulk_title_multi": "New Documents"
 }

frontend/messages/es.json

@@ -872,5 +872,7 @@
 	"bulk_drop_desc": "Se crea un documento separado por archivo. El título se rellena desde el nombre del archivo — el resto de campos se aplican a todos.",
 	"bulk_select_files": "Seleccionar archivos",
 	"bulk_drop_zone_label": "Soltar archivos aquí",
-	"bulk_remove_file": "Eliminar"
+	"bulk_remove_file": "Eliminar",
+	"bulk_title_single": "Nuevo Documento",
+	"bulk_title_multi": "Nuevos Documentos"
 }