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

app.wxss

@@ -110,18 +110,6 @@ page {
   70% { transform: translateY(-6rpx); }
 }
 
-/* ---- shimmer ---- */
-@keyframes shimmer {
-  0% { background-position: -200% 0; }
-  100% { background-position: 200% 0; }
-}
-
-/* ---- spin ---- */
-@keyframes spin {
-  from { transform: rotate(0deg); }
-  to { transform: rotate(360deg); }
-}
-
 .section-title {
   font-size: 32rpx;
   font-weight: 600;
@@ -129,18 +117,38 @@ page {
   margin-bottom: 20rpx;
 }
 
-/* ---- icon sizing ---- */
-.icon {
-  line-height: 1;
-  flex-shrink: 0;
+/* ---- shared modal overlay ---- */
+.modal-overlay {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background: rgba(0, 0, 0, 0.45);
+  z-index: 1000;
 }
 
-.icon-lg { font-size: 40rpx; }
-.icon-md { font-size: 28rpx; }
-.icon-sm { font-size: 22rpx; }
+.modal-overlay-bottom {
+  display: flex;
+  align-items: flex-end;
+  justify-content: center;
+}
 
-.icon-inline {
-  display: inline-flex;
+.modal-overlay-center {
+  display: flex;
   align-items: center;
-  margin-right: 8rpx;
+  justify-content: center;
 }
+
+/* ---- shared danger button ---- */
+.btn-danger {
+  background: #fff;
+  color: #E53935;
+  border: 2rpx solid rgba(229, 57, 53, 0.3);
+  border-radius: 48rpx;
+  font-size: 28rpx;
+  padding: 20rpx 0;
+  text-align: center;
+}
+
+.btn-danger::after { border: none; }