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/main.py

@@ -50,6 +50,18 @@ async def init_db():
     except Exception as e:
         print(f"Column migration skipped: {e}")
 
+    # health_metrics columns added after initial creation
+    try:
+        async with engine.begin() as conn:
+            for stmt in [
+                "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS avg_hr_day FLOAT",
+                "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS max_hr_day FLOAT",
+                "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS intraday_hr JSONB",
+            ]:
+                await conn.execute(text(stmt))
+    except Exception as e:
+        print(f"health_metrics column migration skipped: {e}")
+
     # Replace the all-columns unique constraint on personal_records with a partial
     # index (only current records must be unique per user/sport/distance).
     # The old constraint also covered is_current_record=False rows, causing