Fix uncategorize quiz — omit category_id param when null

Previously sent ?category_id= (empty string) which FastAPI rejects
with 422 validation error when parsing as int.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/QuizzesPage.jsx

@@ -229,7 +229,8 @@ function QuizCard({ quiz, isModerator, categories, onDelete, onCategoryChange })
 
   const assignCategory = async (catId) => {
     try {
-      await api.patch(`/categories/quizzes/${quiz.id}`, null, { params: { category_id: catId ?? '' } })
+      const params = catId == null ? {} : { category_id: catId }
+      await api.patch(`/categories/quizzes/${quiz.id}`, null, { params })
       onCategoryChange(quiz.id, catId)
     } catch { }
     setShowCatMenu(false)