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>