feat(dropzone): add filename hint showing supported naming pattern

Shows a concrete example (2024-03-15_Mueller_Hans.pdf) so users know
which filenames will be auto-parsed during bulk upload.

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

frontend/messages/de.json

@@ -270,6 +270,7 @@
 	"pdf_annotations_hide": "Annotierungen verbergen",
 	"upload_drop_hint": "Dateien ablegen oder auswählen",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
+	"upload_filename_hint": "Tipp: 2024-03-15_Mueller_Hans.pdf → Datum und Absender werden vorausgefüllt",
 	"upload_success": "{count} Dokument(e) erstellt",
 	"upload_duplicate": "{filename} existiert bereits —",
 	"upload_duplicate_link": "Zum Dokument",

frontend/messages/en.json

@@ -270,6 +270,7 @@
 	"pdf_annotations_hide": "Hide annotations",
 	"upload_drop_hint": "Drop files or click to select",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
+	"upload_filename_hint": "Tip: 2024-03-15_Mueller_Hans.pdf → date and sender pre-filled",
 	"upload_success": "{count} document(s) created",
 	"upload_duplicate": "{filename} already exists —",
 	"upload_duplicate_link": "View document",

frontend/messages/es.json

@@ -270,6 +270,7 @@
 	"pdf_annotations_hide": "Ocultar anotaciones",
 	"upload_drop_hint": "Soltar archivos o hacer clic para seleccionar",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
+	"upload_filename_hint": "Consejo: 2024-03-15_Mueller_Hans.pdf → fecha y remitente prellenados",
 	"upload_success": "{count} documento(s) creado(s)",
 	"upload_duplicate": "{filename} ya existe —",
 	"upload_duplicate_link": "Ver documento",