refactor(ui): 12-point UI polish and bugfixes

Bug fixes:
- #1 Remove duplicate month label from calendar-heatmap (shown twice in records)
- #2 Extract shared .modal-overlay to app.wxss (was duplicated in index/records)

UX improvements:
- #3 Add long-press hint to history list in records page
- #4 Add drag handle + catchtouchmove to picker modal
- #5 Extend timer auto-navigate-back from 1.5s to 3s
- #6 Add clear data option in settings with confirmation dialog

Visual/code polish:
- #7 Remove 4 unused @keyframes (shimmer, spin) and 5 unused .icon classes
- #8 Tab bar inactive colors: hardcoded #888/#666 -> var(--text-secondary)
- #10 Split progress-ring observer to avoid redundant setData on size/color changes
- #12 Remove meaningless Math.min(planDay, 99) cap

pages/records/records.wxml

@@ -40,6 +40,7 @@
   <view class="history-section card">
     <view class="history-header">
       <text class="section-title">📋 最近记录</text>
+      <text class="history-hint" wx:if="{{historyList.length > 0}}">长按可删除</text>
     </view>
     <view wx:if="{{historyList.length === 0}}" class="empty-state">
       <text class="empty-emoji">🏃</text>
@@ -63,7 +64,7 @@
   </view>
 
   <!-- 日期详情弹窗 -->
-  <view class="modal-overlay" wx:if="{{showDayDetail}}" bindtap="onCloseDayDetail">
+  <view class="modal-overlay modal-overlay-center" wx:if="{{showDayDetail}}" bindtap="onCloseDayDetail">
     <view class="day-detail-modal" catchtap="noop">
       <view class="detail-date-row">
         <text class="detail-emoji">📅</text>