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/templates/partials/today_content.html

@@ -3,6 +3,19 @@
   🎉 <strong>{{ celebrate_habit_name }}</strong> {{ celebrate_streak }}일 연속 달성! 축하해요!
 </div>
 {% endif %}
+{% if celebrate_level_up %}
+<div class="celebration-banner" x-data="{ show: true }" x-init="setTimeout(() => show = false, 4000)" x-show="show" x-transition>
+  ⭐ <strong>{{ celebrate_habit_name }}</strong> 레벨 {{ celebrate_level_up }}로 올랐어요!
+</div>
+{% endif %}
+
+<div class="card account-level-card">
+  <span class="badge level-badge">{{ level_tier_emoji(account_level.level) }} 계정 Lv.{{ account_level.level }}</span>
+  <div class="xp-bar-wrap">
+    <div class="xp-bar"><div class="xp-bar-fill" style="width: {{ account_level.progress_pct }}%;"></div></div>
+    <span class="xp-bar-label">{{ account_level.xp_into_level }}/{{ account_level.xp_for_next_level }} xp</span>
+  </div>
+</div>
 
 <div
   class="card"