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/services/level_service.py

@@ -0,0 +1,48 @@
+from app.schemas.level import LevelInfo
+
+# 체크 1회당 얻는 기본 경험치.
+XP_PER_CHECK = 10
+
+# 예정일을 놓쳤을 때의 페널티. 연속으로 놓칠수록 (miss_streak * MISS_PENALTY_BASE)로 가속되며,
+# MISS_PENALTY_CAP에서 더 이상 커지지 않는다(끝없이 방치된 습관이 비정상적인 값으로 폭주하지 않도록).
+MISS_PENALTY_BASE = 8
+MISS_PENALTY_CAP = 80
+
+# 연속 달성일 마일스톤(log_service.MILESTONE_STREAKS와 동일한 값) 도달 시 추가로 얹어주는 보너스 경험치.
+STREAK_MILESTONE_BONUS_XP: dict[int, int] = {
+    7: 20,
+    30: 80,
+    66: 150,
+    100: 250,
+    200: 500,
+    365: 1000,
+}
+
+# 레벨업에 필요한 경험치는 LEVEL_BASE_XP에서 시작해 레벨마다 LEVEL_STEP_XP씩 늘어난다
+# (초반엔 빠르게, 갈수록 완만하게 오르는 곡선).
+LEVEL_BASE_XP = 20
+LEVEL_STEP_XP = 10
+
+
+def _xp_required_for_level(level: int) -> int:
+    """`level`에서 `level + 1`로 올라가는 데 필요한 경험치."""
+    return LEVEL_BASE_XP + LEVEL_STEP_XP * (level - 1)
+
+
+def level_from_xp(xp: int) -> LevelInfo:
+    xp = max(xp, 0)
+    level = 1
+    remaining = xp
+    while True:
+        needed = _xp_required_for_level(level)
+        if remaining < needed:
+            progress_pct = round(remaining / needed * 100, 1)
+            return LevelInfo(
+                xp=xp,
+                level=level,
+                xp_into_level=remaining,
+                xp_for_next_level=needed,
+                progress_pct=progress_pct,
+            )
+        remaining -= needed
+        level += 1