style: 学习任务统计区改为卡片网格

src/components/Study.vue

@@ -662,7 +662,7 @@ onMounted(() => {
 
 .stats-grid {
   display: grid;
-  grid-template-columns: repeat(2, 1fr);
+  grid-template-columns: repeat(4, 1fr);
   gap: 12px;
 }
 
@@ -670,17 +670,27 @@ onMounted(() => {
   display: flex;
   flex-direction: column;
   gap: 4px;
+  padding: 10px 12px;
+  background: var(--accent-success-softer);
+  border: 1px solid var(--border-soft);
+  border-radius: 8px;
 }
 
 .stat-label {
-  font-size: 13px;
+  font-size: 12px;
   color: var(--text-secondary);
 }
 
 .stat-value {
-  font-size: 15px;
+  font-size: 16px;
   font-weight: 600;
-  color: var(--text-primary);
+  color: var(--green-900);
+}
+
+@media (max-width: 900px) {
+  .stats-grid {
+    grid-template-columns: repeat(2, 1fr);
+  }
 }
 
 .application-list {