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/habit_item.html

@@ -35,6 +35,11 @@
             {{ '🏆' if is_milestone_streak(stats.current_streak) else '🔥' }} 연속 {{ stats.current_streak }}일
           </span>
           {% endif %}
+          <span class="badge level-badge">{{ level_tier_emoji(stats.level_info.level) }} Lv.{{ stats.level_info.level }}</span>
+        </div>
+        <div class="xp-bar-wrap">
+          <div class="xp-bar"><div class="xp-bar-fill" style="width: {{ stats.level_info.progress_pct }}%;"></div></div>
+          <span class="xp-bar-label">{{ stats.level_info.xp_into_level }}/{{ stats.level_info.xp_for_next_level }} xp</span>
         </div>
         {% endif %}
       </div>