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_item.html

@@ -20,6 +20,11 @@
           {{ '🏆' if is_milestone_streak(item.current_streak) else '🔥' }} 연속 {{ item.current_streak }}일
         </span>
         {% endif %}
+        <span class="badge level-badge">{{ level_tier_emoji(item.level_info.level) }} Lv.{{ item.level_info.level }}</span>
+      </div>
+      <div class="xp-bar-wrap">
+        <div class="xp-bar"><div class="xp-bar-fill" style="width: {{ item.level_info.progress_pct }}%;"></div></div>
+        <span class="xp-bar-label">{{ item.level_info.xp_into_level }}/{{ item.level_info.xp_for_next_level }} xp</span>
       </div>
       {% endif %}
     </div>