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

custom-tab-bar/index.wxss

@@ -28,7 +28,7 @@
 
 .tab-text {
   font-size: 24rpx;
-  color: #666;
+  color: var(--text-secondary);
   margin-top: 8rpx;
   line-height: 1;
 }
@@ -54,7 +54,7 @@
   height: 0;
   border-left: 22rpx solid transparent;
   border-right: 22rpx solid transparent;
-  border-bottom: 18rpx solid #888;
+  border-bottom: 18rpx solid var(--text-secondary);
   position: absolute;
   top: 0;
   left: 0;
@@ -67,7 +67,7 @@
 .home-body {
   width: 22rpx;
   height: 20rpx;
-  background: #888;
+  background: var(--text-secondary);
   position: absolute;
   bottom: 0;
   left: 11rpx;
@@ -94,7 +94,7 @@
 
 .bar {
   height: 6rpx;
-  background: #888;
+  background: var(--text-secondary);
   border-radius: 3rpx;
 }
 
@@ -120,7 +120,7 @@
 .gear-outer {
   width: 36rpx;
   height: 36rpx;
-  border: 5rpx solid #888;
+  border: 5rpx solid var(--text-secondary);
   border-radius: 50%;
   position: absolute;
   top: 3rpx;
@@ -134,7 +134,7 @@
 .gear-center {
   width: 12rpx;
   height: 12rpx;
-  background: #888;
+  background: var(--text-secondary);
   border-radius: 50%;
   position: absolute;
   top: 15rpx;