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/main.py

@@ -57,6 +57,7 @@ async def init_db():
                 "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",
+                "ALTER TABLE health_metrics ADD COLUMN IF NOT EXISTS body_battery JSONB",
             ]:
                 await conn.execute(text(stmt))
     except Exception as e: