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/schemas/level.py

@@ -0,0 +1,9 @@
+from pydantic import BaseModel
+
+
+class LevelInfo(BaseModel):
+    xp: int  # 누적 경험치 (0 이상)
+    level: int  # 현재 레벨 (1부터 시작)
+    xp_into_level: int  # 현재 레벨 진입 후 쌓인 xp
+    xp_for_next_level: int  # 다음 레벨까지 필요한 xp
+    progress_pct: float  # 다음 레벨까지 진행률 (0~100)