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>

components/trend-chart/trend-chart.wxss

@@ -0,0 +1,68 @@
+.trend-chart {
+  /* top room so the value labels above tall bars aren't clipped */
+  padding-top: 32rpx;
+}
+
+.trend-bars {
+  display: flex;
+  align-items: flex-end;
+  height: 220rpx;
+  gap: 14rpx;
+}
+
+.trend-bar-col {
+  flex: 1;
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+  justify-content: flex-end;
+}
+
+.trend-bar {
+  width: 100%;
+  position: relative;
+  border-radius: 8rpx 8rpx 0 0;
+  background: linear-gradient(180deg, var(--primary), var(--primary-light));
+  min-height: 6rpx;
+  transition: height 0.4s cubic-bezier(0.4, 0, 0.2, 1);
+}
+
+.trend-bar.zero {
+  background: var(--border);
+  min-height: 4rpx;
+}
+
+.trend-bar.highlight {
+  box-shadow: 0 0 14rpx rgba(var(--primary-rgb), 0.5);
+}
+
+/* value caption floats above the bar so it never eats into bar height */
+.trend-bar-value {
+  position: absolute;
+  top: -28rpx;
+  left: 0;
+  right: 0;
+  text-align: center;
+  font-size: 18rpx;
+  color: var(--text-secondary);
+  line-height: 1;
+  white-space: nowrap;
+}
+
+.trend-labels {
+  display: flex;
+  margin-top: 12rpx;
+  gap: 14rpx;
+}
+
+.trend-bar-label {
+  flex: 1;
+  text-align: center;
+  font-size: 20rpx;
+  color: var(--text-secondary);
+}
+
+.trend-bar-label.highlight {
+  color: var(--primary);
+  font-weight: 600;
+}