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/static/css/style.css

@@ -409,6 +409,40 @@ label {
   color: var(--color-accent);
 }
 
+/* 목표 기간 난이도 선택 */
+.difficulty-picker {
+  display: flex;
+  gap: 6px;
+  flex-wrap: wrap;
+}
+
+.difficulty-pill {
+  border-radius: 999px;
+  border: 1px solid var(--color-border);
+  background: transparent;
+  color: var(--color-text-muted);
+  font-size: 13px;
+  padding: 8px 14px;
+  cursor: pointer;
+}
+
+.difficulty-pill.selected {
+  background: var(--color-accent);
+  border-color: var(--color-accent);
+  color: #fff;
+}
+
+.difficulty-hint {
+  font-size: 12px;
+  color: var(--color-text-muted);
+  margin-top: 6px;
+}
+
+.goal-progress-badge {
+  font-size: 12px;
+  color: var(--color-text-muted);
+}
+
 /* 습관 리스트 아이템 */
 .habit-item {
   display: flex;