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.wxss

@@ -79,6 +79,28 @@
   color: var(--primary);
 }
 
+/* ---- trend ---- */
+.trend-section {
+  padding: 0;
+}
+
+.trend-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-bottom: 8rpx;
+}
+
+.trend-title-wrap {
+  display: flex;
+  align-items: center;
+  gap: 8rpx;
+}
+
+.trend-header .section-title {
+  margin-bottom: 0;
+}
+
 /* ---- history ---- */
 .history-section {
   padding: 0;