fix(bulk-upload): PDF-only file acceptance

Drop non-PDF accept types from file input and update format hint strings
in all three languages. JPEG/PNG/TIFF were never officially supported.

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

frontend/src/lib/components/document/BulkDropZone.svelte

@@ -68,7 +68,7 @@ let isDragging = $state(false);
 			<input
 				type="file"
 				multiple
-				accept="application/pdf,image/jpeg,image/png,image/tiff"
+				accept="application/pdf"
 				class="sr-only"
 				onchange={(e) => {
 					const files = Array.from(e.currentTarget.files ?? []);