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/tests/test_health.py

@@ -119,12 +119,17 @@ class TestUpsert:
         b = health_svc.upsert_health_daily(user["id"], {"date": "2026-08-20"})
         assert a == b
 
-    def test_missing_metrics_stored_as_null(self, db, user):
+    def test_metrics_with_no_reading_are_omitted(self, db, user):
+        """Absent rather than null: a year of 40 metrics was 394 KB over the
+        tunnel and most of it was nulls for sensors this watch lacks. The UI
+        reads a missing key and a null the same way."""
         health_svc.upsert_health_daily(
             user["id"], {"date": "2026-08-20", "steps": 100}
         )
         row = health_svc.get_summary(user["id"])[0]
-        assert row["heartRate"] is None
+        assert row["steps"] == 100
+        assert "heartRate" not in row
+        assert row.get("heartRate") is None, "reading it must still be falsy"
         assert row["sleep"] is None