feat(data): 体重差值按 weightDiff/weightTrend 动态渲染并区分增减颜色

- 差值图标与数值改用后端 weightDiff/weightTrend
- 首次测量/持平不显示差值块
- 增加红色、减少绿色

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

miniprogram/pages/data/data.ts

@@ -38,6 +38,10 @@ interface MeasureRecord {
     weight: number
     bmi: number
     fatRate: number
+    /** 相比上次体重差值绝对值(kg)，首次测量为 null */
+    weightDiff: number | null
+    /** 变化方向：1增加 -1减少 0持平 null首次 */
+    weightTrend: number | null
 }
 
 /** GET /weighingScale/v3/getMeasureHistoryPage 返回结构 */
@@ -150,6 +154,8 @@ Page({
                     weight: r.weight ?? 0,
                     bmi: r.bmi ?? 0,
                     fatRate: r.fatRate ?? 0,
+                    weightDiff: r.weightDiff == null ? null : Math.abs(r.weightDiff),
+                    weightTrend: r.weightTrend ?? null,
                 }))
                 const total = page.total ?? 0
 
@@ -191,6 +197,8 @@ Page({
                     weight: r.weight ?? 0,
                     bmi: r.bmi ?? 0,
                     fatRate: r.fatRate ?? 0,
+                    weightDiff: r.weightDiff == null ? null : Math.abs(r.weightDiff),
+                    weightTrend: r.weightTrend ?? null,
                 }))
                 const allRecords = [...this.data.records, ...newRecords]
                 this.setData({