fix: align difficulty statistics on homepage with challenge list filters

Author: CC11001100

src/components/HomePage/index.tsx

@@ -11,9 +11,11 @@ import { useMediaQuery } from 'react-responsive';
 const getDifficultyCounts = () => {
   const counts = { easy: 0, medium: 0, hard: 0 };
   challenges.forEach(challenge => {
-    if (challenge.difficulty === 1) counts.easy++;
-    else if (challenge.difficulty === 2) counts.medium++;
-    else if (challenge.difficulty === 3) counts.hard++;
+    // 匹配挑战列表页面的难度分类:
+    // easy: 1-2星, medium: 3-4星, hard: 5星
+    if (challenge.difficulty <= 2) counts.easy++;
+    else if (challenge.difficulty <= 4) counts.medium++;
+    else if (challenge.difficulty === 5) counts.hard++;
   });
   return counts;
 };