add habit leveling/XP system, explicit fail marking for missed habits, and scrollable habit tabs

- Habits and the account now accumulate XP and levels from check streaks, with
  streak/level-up celebration banners and push notifications.
- The "yesterday missed" banner on /today now lets a habit be explicitly marked
  failed (not just checked done), backed by a new HabitLog.status column so
  completion-rate/streak calculations never count a failed day as done.
- The habit management tabs scroll horizontally on narrow screens instead of
  wrapping to two lines, with a pure-CSS edge shadow indicating more content.

app/routers/logs.py

@@ -23,8 +23,8 @@ def toggle_log(habit_id: int, db: Session = Depends(get_db), current_user: User
     habit = habit_service.get_habit(db, habit_id, current_user.id)
     if habit is None:
         raise HTTPException(status_code=404, detail="습관을 찾을 수 없습니다")
-    checked, milestone_streak = log_service.toggle_check_and_celebrate(db, habit, date.today())
-    return {"checked": checked, "milestone_streak": milestone_streak}
+    checked, milestone_streak, level_up = log_service.toggle_check_and_celebrate(db, habit, date.today())
+    return {"checked": checked, "milestone_streak": milestone_streak, "level_up": level_up}
 
 
 @router.get("/logs", response_model=list[HabitLogOut])