Show existing image preview when editing an image question

Set imagePreview to the question's image_path in handleEdit so the
current image displays in the Sheet form.

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

frontend/frontend/src/components/questionbank/QuestionBankView.tsx

@@ -291,6 +291,7 @@ export default function QuestionBankView() {
 			start_time: 0,
 			end_time: 0,
 		});
+		setImagePreview(question.image_path || null);
 		setSheetOpen(true);
 	};