Cut Garmin sync API volume; dashboard/health/records/UI improvements

Garmin Connect sync:
- Incremental syncs now re-fetch only a 1-day buffer (yesterday + today)
  instead of the full lookback window every run. Full lookback applies on
  the first sync only. Cuts steady-state API calls ~10x.
- Beat interval is now configurable via GARMIN_SYNC_INTERVAL_MINUTES and
  surfaced to the UI; the sync toggle is relabelled to the real cadence.

Frontend:
- Collapsible sidebar; clearer logged-in user + role display.
- Unified Body Battery colouring between dashboard and health (shared util).
- Sleep score trend chart on health page.
- Segments + medals on the dashboard's most-recent activity.
- Segments tab on the Records page.

Repo hygiene: add .gitignore, untrack committed __pycache__/*.pyc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/workers/tasks.py

@@ -25,7 +25,8 @@ celery_app.conf.update(
     beat_schedule={
         "sync-garmin-connect": {
             "task": "sync_all_garmin_connect",
-            "schedule": 1800.0,  # every 30 minutes
+            # Interval is configurable via GARMIN_SYNC_INTERVAL_MINUTES (default 30 min)
+            "schedule": float(settings.garmin_sync_interval_minutes * 60),
         },
     },
 )