Fix missing avg_hr_day/weight data; add 24hr HR chart to daily snapshot

Backend:
- main.py: add ADD COLUMN IF NOT EXISTS migrations for avg_hr_day, max_hr_day,
  and intraday_hr (JSONB) on health_metrics — these columns were in the model
  but missing from existing DB instances, silently dropping all avg/max HR data.
- models/user.py: add intraday_hr JSON column to HealthMetric.
- garmin_connect_sync.py: fetch body composition (weight, BMI, body fat, muscle
  mass) via get_body_composition() per day, with stats.bodyWeight as fallback.
  Fetch intraday heart rate via get_heart_rates() and store non-null
  [epoch_ms, bpm] pairs in intraday_hr.
- health.py: add GET /health-metrics/intraday?date=YYYY-MM-DD endpoint that
  returns the stored intraday_hr array for a specific day.

Frontend (HealthPage):
- Add IntradayHrChart component: AreaChart rendering the 24-hour HR trace
  with time-of-day x-axis.
- DailySnapshot: show 24-hour HR chart (when intraday data present) above
  the activity strip; add weight + body fat % to the Heart & HRV card;
  show max HR alongside avg HR.
- HealthPage: query /intraday for the selected day and pass data down.

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

backend/app/api/health.py

@@ -115,6 +115,30 @@ async def health_summary(
     }
 
 
+@router.get("/intraday")
+async def intraday_health(
+    date: str = Query(..., description="YYYY-MM-DD"),
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Return intraday heart rate series for a specific day."""
+    from datetime import date as _date
+    from fastapi import HTTPException
+    try:
+        metric_date = _date.fromisoformat(date)
+    except ValueError:
+        raise HTTPException(status_code=400, detail="date must be YYYY-MM-DD")
+
+    result = await db.execute(
+        select(HealthMetric).where(
+            HealthMetric.user_id == current_user.id,
+            func.date(HealthMetric.date) == metric_date,
+        )
+    )
+    metric = result.scalar_one_or_none()
+    return {"hr_values": metric.intraday_hr if metric else None}
+
+
 @router.put("/manual")
 async def add_manual_metric(
     body: dict,