perf: 今日页首屏 394KB/2.0s → 51.5KB/0.82s

逐个接口计时后的两处改动：

- 健康摘要不再下发空值。一天 40 项指标里大部分是这块表没有的传感器，
  全按 null 发出去占了约两成体积。客户端本来就把「键不存在」和 null
  当同一回事。
- 今日页首屏由 365 天改为 60 天，往前翻越界时再加载 180 天。
  一次取一年是为了让翻页不发请求，代价是首屏 394 KB，手机上不划算。
  日历选到窗口外的日期同样会自动加载。

顺带把加载逻辑收成一个 loadFrom：原来初始 effect 的依赖是空数组，
日历里改 from 不会触发重新拉取，是个还没被触发的 bug。

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/services/health.py

@@ -37,7 +37,13 @@ def get_summary(user_id, start=None, end=None):
     for r in rows:
         day = {"date": r["date"]}
         for column, key in HEALTH_COLUMNS.items():
-            day[key] = r.get(column)
+            value = r.get(column)
+            # Null metrics are omitted rather than sent as null. A year of 40
+            # metrics is 394 KB over the tunnel, and most of it is nulls for
+            # sensors this watch does not have; the client already treats a
+            # missing key and a null the same way.
+            if value is not None:
+                day[key] = value
         # Sleep stays nested for backwards compatibility with the UI and the
         # existing recommendation rules.
         day["sleep"] = (