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.wxss

@@ -295,28 +295,23 @@
 }
 
 /* ---- picker modal ---- */
-.modal-overlay {
-  position: fixed;
-  top: 0;
-  left: 0;
-  right: 0;
-  bottom: 0;
-  background: rgba(0, 0, 0, 0.45);
-  z-index: 1000;
-  display: flex;
-  align-items: flex-end;
-  justify-content: center;
-}
-
 .picker-modal {
   width: 100%;
   background: #fff;
   border-radius: 32rpx 32rpx 0 0;
-  padding: 40rpx 32rpx;
+  padding: 28rpx 32rpx 40rpx;
   padding-bottom: calc(40rpx + env(safe-area-inset-bottom));
   animation: slideUp 0.3s cubic-bezier(0.32, 0.72, 0, 1);
 }
 
+.picker-handle {
+  width: 64rpx;
+  height: 8rpx;
+  background: #DDD;
+  border-radius: 4rpx;
+  margin: 0 auto 24rpx;
+}
+
 @keyframes slideUp {
   from { transform: translateY(100%); }
   to { transform: translateY(0); }