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/index/index.wxml

@@ -79,8 +79,9 @@
 </view>
 
 <!-- 时间选择弹窗 -->
-<view class="modal-overlay" wx:if="{{showPicker}}" bindtap="onClosePicker">
-  <view class="picker-modal" catchtap="noop">
+<view class="modal-overlay modal-overlay-bottom" wx:if="{{showPicker}}" bindtap="onClosePicker">
+  <view class="picker-modal" catchtap="noop" catchtouchmove="noop">
+    <view class="picker-handle"></view>
     <text class="picker-title">⏱ 选择训练时长</text>
     <view class="preset-grid">
       <view