fix(autograde): clearer message when no auto-gradable questions exist

For MCQ-only assignments canAutoGrade is false because gradableCount==0,
but missingModelAnswers is also 0 — so the warning rendered as "0
question(s) missing model answers", which is wrong and confusing. Show
"No auto-gradable questions" in that branch instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: WangYuTengg

src/client/features/staff-grading/AssignmentAutoGradingList.tsx

@@ -447,7 +447,9 @@ export function AssignmentAutoGradingList({ courseId }: AssignmentAutoGradingLis
                         <div className="inline-flex items-center gap-2">
                           <ExclamationTriangleIcon className="h-5 w-5 text-yellow-500" />
                           <span className="text-yellow-700 text-xs">
-                            {assignment.missingModelAnswers.length} question(s) missing model answers
+                            {assignment.missingModelAnswers.length > 0
+                              ? `${assignment.missingModelAnswers.length} question(s) missing model answers`
+                              : 'No auto-gradable questions'}
                           </span>
                         </div>
                       ) : assignment.ungradedAnswers === 0 ? (