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_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>