add habit goal-period difficulty and habit formation research notes

Ground the 21-day habit myth in actual research (Lally et al. 2010) and let
users pick a target period (21/66/254 days or unlimited) matching that
study's easy/median/hard automaticity timelines when creating a habit,
with progress shown on the habits list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/partials/habit_item.html

@@ -3,6 +3,7 @@
   x-data="{
     editing: false,
     mask: {{ habit.weekdays_mask }},
+    difficulty: '{{ habit.difficulty.value }}',
     alarmOn: {{ 'true' if habit.reminder_time else 'false' }},
     days: ['월', '화', '수', '목', '금', '토', '일'],
   }"
@@ -17,6 +18,14 @@
           {{ weekday_label(habit.weekdays_mask) }}
           {% if habit.reminder_time %}· 알람 {{ habit.reminder_time.strftime("%H:%M") }}{% endif %}
         </div>
+        {% set progress = goal_progress(habit) %}
+        <div class="goal-progress-badge">
+          {% if progress %}
+          목표 {{ difficulty_label(habit.difficulty) }} · {{ progress.elapsed }}/{{ progress.target }}일차{% if progress.reached %} · 목표 기간 달성!{% else %} (D-{{ progress.remaining }}){% endif %}
+          {% else %}
+          {{ difficulty_label(habit.difficulty) }}
+          {% endif %}
+        </div>
         {% set stats = stats_map[habit.id] %}
         {% if stats.scheduled_days > 0 %}
         <div class="habit-item-stats">
@@ -78,6 +87,7 @@
     hx-swap="innerHTML"
   >
     <input type="hidden" name="weekdays_mask" :value="mask" />
+    <input type="hidden" name="difficulty" :value="difficulty" />
 
     <div class="field">
       <label for="edit-name-{{ habit.id }}">습관 이름</label>