fix(category): 实现触底分页加载更多，修复往下翻无内容

- 根因：category.js 的 loadExercises 永远只取第 1 页(page=1, pageSize=20)，
  且用 exercises = res.items 整体替换、从不追加，也没有 onReachBottom/触底加载。
  动作数远超 20(全部 1324、胸 163)时，往下翻只能看到前 20 条，其余永远加载不出来。
- 修复：
  * loadExercises 首次加载重置 page=1 并按 items.length<total 计算 hasMore
  * 新增 onReachBottom + loadMore：触底时 page+1、concat 追加、刷新 hasMore
  * wxml 在网格下方加 loadmore 状态条(加载中 spinner/已经到底啦/上拉加载更多)
  * wxss 加 .spinner 旋转动画(用 --accent/--line，均在 app.wxss 已定义)
- 后端分页已验证：page=2 返回偏移后的不同数据，total 字段准确

miniprogram/pages/category/category.js

@@ -29,6 +29,8 @@ Page({
     activeValue: '',
     exercises: [],
     loading: true,
+    loadingMore: false,
+    hasMore: true,
     page: 1,
     pageSize: 20,
     total: 0
@@ -67,20 +69,49 @@ Page({
   },
 
   loadExercises() {
-    this.setData({ loading: true });
-    const params = { page: this.data.page, pageSize: this.data.pageSize };
+    this.setData({ loading: true, page: 1 });
+    const params = { page: 1, pageSize: this.data.pageSize };
     params[this.data.dimension] = this.data.activeValue;
     svc.listExercises(params).then((res) => {
+      const items = (res && res.items) || [];
+      const total = (res && res.total) || 0;
       this.setData({
-        exercises: (res && res.items) || [],
-        total: (res && res.total) || 0,
-        loading: false
+        exercises: items,
+        total,
+        loading: false,
+        hasMore: items.length < total
       });
     }).catch(() => {
       this.setData({ loading: false });
     });
   },
 
+  onReachBottom() {
+    if (this.data.loading || this.data.loadingMore || !this.data.hasMore) return;
+    this.loadMore();
+  },
+
+  loadMore() {
+    const next = this.data.page + 1;
+    this.setData({ loadingMore: true });
+    const params = { page: next, pageSize: this.data.pageSize };
+    params[this.data.dimension] = this.data.activeValue;
+    svc.listExercises(params).then((res) => {
+      const items = (res && res.items) || [];
+      const total = (res && res.total) || 0;
+      const merged = this.data.exercises.concat(items);
+      this.setData({
+        exercises: merged,
+        total,
+        page: next,
+        loadingMore: false,
+        hasMore: merged.length < total
+      });
+    }).catch(() => {
+      this.setData({ loadingMore: false });
+    });
+  },
+
   onValue(e) {
     const value = e.detail.value;
     if (value === this.data.activeValue) return;