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>

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"

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>

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>