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/settings/settings.js

@@ -98,5 +98,21 @@ Page({
         wx.showToast({ title: '已复制', icon: 'success', duration: 1500 })
       }
     })
+  },
+
+  onClearData() {
+    wx.showModal({
+      title: '清除数据',
+      content: '将删除所有训练记录和连续打卡数据，此操作不可恢复。确定继续吗？',
+      confirmText: '确定清除',
+      confirmColor: '#E53935',
+      success: (res) => {
+        if (res.confirm) {
+          wx.removeStorageSync('training_records')
+          wx.removeStorageSync('current_streak')
+          wx.showToast({ title: '已清除', icon: 'success', duration: 1500 })
+        }
+      }
+    })
   }
 })