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.json

@@ -1,6 +1,7 @@
 {
   "usingComponents": {
     "calendar-heatmap": "/components/calendar-heatmap/calendar-heatmap",
+    "trend-chart": "/components/trend-chart/trend-chart",
     "ui-card": "/components/ui-card/ui-card"
   },
   "navigationBarTitleText": "训练记录"