Add body battery: sync, storage, and health UI chart

Parses Garmin Connect get_body_battery() per day, storing charged/drained/
start+end levels and the fine-grained [[ts_ms, level, type, stress]] values
array in a new body_battery JSONB column on health_metrics.

Frontend adds:
- BatteryRing SVG gauge (color-scaled 0–100)
- BodyBatteryChart: ComposedChart with type-colored bars (REST/ACTIVE/SLEEP/
  STRESS) and battery level overlay line, matching Garmin's layout
- Body battery trend chart in the Trends section (end_level per day)

Also adds avg_hr_day and weight data which now correctly sync with the
intraday_hr JSON serialization fix from the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/user.py

@@ -244,6 +244,7 @@ class HealthMetric(Base):
     total_calories = Column(Float, nullable=True)
     spo2_avg = Column(Float, nullable=True)
     intraday_hr = Column(JSON, nullable=True)   # [[epoch_ms, bpm], ...] — not in API list response
+    body_battery = Column(JSON, nullable=True)  # {charged,drained,start_level,end_level,values:[[ts_ms,level,type,stress]...]}
 
     __table_args__ = (
         UniqueConstraint("user_id", "date", name="uq_health_user_date"),