feat(dropzone): replace upload icon with multi-file icon and clearer hint text

Swaps the generic upload arrow for Display-Pages-MD (stack of pages) and
shortens the hint text to convey that multiple files are welcome at a glance.

Closes #79
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -268,7 +268,7 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}",
 	"pdf_annotations_show": "Annotierungen anzeigen",
 	"pdf_annotations_hide": "Annotierungen verbergen",
-	"upload_drop_hint": "Dateien ablegen oder auswählen",
+	"upload_drop_hint": "Einzeln oder mehrere Dateien auf einmal",
 	"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",