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

components/progress-ring/progress-ring.js

@@ -17,12 +17,15 @@ Component({
   },
 
   observers: {
-    'remaining, status, duration, size, ringWidth, primaryColor'() {
+    'remaining, status, duration'() {
       const texts = { idle: '准备开始', running: '坚持住', paused: '已暂停', completed: '完成!' }
       this.setData({
         displayTime: util.formatTime(this.data.remaining),
         statusText: texts[this.data.status] || ''
       })
+    },
+
+    'remaining, duration, size, ringWidth, primaryColor'() {
       this._draw()
     }
   },