fix(layout): replace hardcoded 'Hochladen' with m.upload_action() + aria-label

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

frontend/messages/de.json

@@ -363,6 +363,7 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}",
 	"pdf_annotations_show": "Annotierungen anzeigen",
 	"pdf_annotations_hide": "Annotierungen verbergen",
+	"upload_action": "Hochladen",
 	"upload_drop_hint": "Einzeln oder mehrere Dateien auf einmal hochladen",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
 	"upload_filename_hint": "Tipp: 2024-03-15_Mueller_Hans.pdf → Datum und Absender werden vorausgefüllt",

frontend/messages/en.json

@@ -363,6 +363,7 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}",
 	"pdf_annotations_show": "Show annotations",
 	"pdf_annotations_hide": "Hide annotations",
+	"upload_action": "Upload",
 	"upload_drop_hint": "Drop one or multiple files at once",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
 	"upload_filename_hint": "Tip: 2024-03-15_Mueller_Hans.pdf → date and sender pre-filled",

frontend/messages/es.json

@@ -363,6 +363,7 @@
 	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}",
 	"pdf_annotations_show": "Mostrar anotaciones",
 	"pdf_annotations_hide": "Ocultar anotaciones",
+	"upload_action": "Subir",
 	"upload_drop_hint": "Uno o varios archivos a la vez",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
 	"upload_filename_hint": "Consejo: 2024-03-15_Mueller_Hans.pdf → fecha y remitente prellenados",