feat(records): 训练趋势图表 + 列表时长改 MM:SS

- 新增 trend-chart 柱状图组件(CSS,近7天/近6月切换)
- records 页加训练趋势卡片(统计卡片后)
- 训练记录列表时长从秒数改为 MM:SS 显示

Co-Authored-By: Claude <noreply@anthropic.com>

pages/records/records.wxml

@@ -44,6 +44,23 @@
     </view>
   </ui-card>
 
+  <!-- 训练趋势 -->
+  <ui-card variant="in" hidden="{{totalSessions === 0}}">
+    <view class="trend-section">
+      <view class="trend-header">
+        <view class="trend-title-wrap">
+          <image class="history-title-icon" src="{{icons.targetFill}}" mode="aspectFit"></image>
+          <text class="section-title">训练趋势</text>
+        </view>
+        <view class="rec-seg">
+          <view class="rec-seg-item {{trendMode === 'week' ? 'active' : ''}}" data-mode="week" bindtap="onToggleTrendMode">近7天</view>
+          <view class="rec-seg-item {{trendMode === 'month' ? 'active' : ''}}" data-mode="month" bindtap="onToggleTrendMode">近6月</view>
+        </view>
+      </view>
+      <trend-chart chart-data="{{trendData}}"></trend-chart>
+    </view>
+  </ui-card>
+
   <!-- 日历热力图 -->
   <ui-card variant="in" hidden="{{totalSessions === 0}}">
     <view class="calendar-section">
@@ -104,7 +121,7 @@
             <text class="history-plan">{{item.day === 0 ? '自由训练' : '第 ' + item.day + ' 天'}}</text>
           </view>
           <view class="history-right">
-            <text class="history-duration">{{item.duration}}s</text>
+            <text class="history-duration">{{item.durationText}}</text>
             <view class="history-delete-btn" catch:tap="onDeleteRecord" data-id="{{item.id}}">
               <image class="history-delete-icon" src="{{icons.deleteActive}}" mode="aspectFit"></image>
               <text class="history-delete-text">删除</text>