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/records/records.wxss

@@ -78,9 +78,21 @@
 
 /* ---- history ---- */
 .history-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
   margin-bottom: 8rpx;
 }
 
+.history-header .section-title {
+  margin-bottom: 0;
+}
+
+.history-hint {
+  font-size: 22rpx;
+  color: #CCC;
+}
+
 .empty-state {
   display: flex;
   flex-direction: column;
@@ -150,19 +162,6 @@
 }
 
 /* ---- day detail popup ---- */
-.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: center;
-  justify-content: center;
-}
-
 .day-detail-modal {
   width: 560rpx;
   background: #fff;