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/timer/timer.js

@@ -41,7 +41,7 @@ Page({
       duration: target,
       remaining: target,
       todayTarget: target,
-      planDay: isFreeMode ? 0 : Math.min(planDay, 99),
+      planDay: isFreeMode ? 0 : planDay,
       isFreeMode
     })
 
@@ -105,6 +105,6 @@ Page({
     storage.updateStreak(today)
 
     wx.showToast({ title: `已记录 ${elapsed}秒`, icon: 'none', duration: 1500 })
-    setTimeout(() => { wx.navigateBack() }, 1500)
+    setTimeout(() => { wx.navigateBack() }, 3000)
   }
 })