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/template_utils.py

@@ -17,6 +17,16 @@ def is_milestone_streak(streak: int) -> bool:
     return streak in MILESTONE_STREAKS
 
 
+def level_tier_emoji(level: int) -> str:
+    if level >= 20:
+        return "💎"
+    if level >= 10:
+        return "⭐"
+    if level >= 5:
+        return "🔥"
+    return "🌱"
+
+
 def weekday_label(mask: int) -> str:
     if mask == ALL_WEEKDAYS_MASK:
         return "매일"