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/yesterday_item.html

@@ -13,4 +13,14 @@
       {% if item.condition_text %}<div class="habit-item-condition">{{ item.condition_text }}</div>{% endif %}
     </div>
   </div>
+  <div class="habit-item-actions">
+    <button
+      type="button"
+      class="btn btn-danger-ghost"
+      hx-post="/today/{{ item.habit_id }}/fail-yesterday"
+      hx-target="#today-content"
+      hx-swap="innerHTML"
+      aria-label="{{ item.name }} 어제 실패로 표시"
+    >실패로 표시</button>
+  </div>
 </div>