HRV balanced dots, dashed gap lines, dashboard widgets + drag-to-edit layout

- Green dots for balanced HRV (joining orange unbalanced / red low) on the trend
- Trend charts bridge data gaps with a dashed line instead of a blank break
- Dashboard: drop Health today; add VO2 max, small sleep, and HRV status widgets
- Dashboard: editable widget grid (react-grid-layout) with drag/resize, saved per-user

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

backend/app/main.py

@@ -100,6 +100,15 @@ async def init_db():
     except Exception as e:
         print(f"users.goal_weight_kg column migration skipped: {e}")
 
+    # dashboard_layout column on users added after initial creation
+    try:
+        async with engine.begin() as conn:
+            await conn.execute(text(
+                "ALTER TABLE users ADD COLUMN IF NOT EXISTS dashboard_layout JSON"
+            ))
+    except Exception as e:
+        print(f"users.dashboard_layout column migration skipped: {e}")
+
     # Backfill avg_hr_day / max_hr_day from intraday_hr for Garmin Connect synced days
     try:
         async with engine.begin() as conn: