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/_weekday_alarm_fields.html

@@ -26,3 +26,18 @@
   </label>
   <input x-show="alarmOn" x-cloak type="time" name="reminder_time" value="{{ reminder_value|default('') }}" style="margin-top:6px;" />
 </div>
+
+<div class="field">
+  <label>목표 기간 난이도</label>
+  <div class="difficulty-picker">
+    {% for d in habit_difficulties %}
+    <button
+      type="button"
+      class="difficulty-pill"
+      :class="{ selected: difficulty === '{{ d.value }}' }"
+      @click="difficulty = '{{ d.value }}'"
+    >{{ difficulty_label(d) }}</button>
+    {% endfor %}
+  </div>
+  <p class="difficulty-hint">습관 자동화까지 걸리는 기간에 대한 실증 연구(Lally et al., 2010, UCL) 기준입니다 — 하는 단순한 습관이 자동화되는 데 걸리는 기간(약 3주), 중은 전체 중앙값, 상은 노력이 많이 드는 습관의 상한(약 8개월)입니다.</p>
+</div>

app/templates/partials/habit_form.html

@@ -3,6 +3,7 @@
   x-data="{
     open: false,
     mask: 127,
+    difficulty: 'medium',
     alarmOn: false,
     days: ['월', '화', '수', '목', '금', '토', '일'],
   }"
@@ -22,6 +23,7 @@
   >
     <input type="hidden" name="habit_type" value="{{ habit_type }}" />
     <input type="hidden" name="weekdays_mask" :value="mask" />
+    <input type="hidden" name="difficulty" :value="difficulty" />
 
     <div class="field">
       <label for="name-{{ habit_type }}">습관 이름</label>

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>