fix(login-sheet): overlay z-index 回退 99999 恢复全库唯一置顶不变量（P1）

cfc-frontend/components/login-sheet.vue

@@ -337,7 +337,7 @@ $ls-dim-wealth-tint: $dim-wealth-tint; // 富浅底 #FFFBEB
   right: 0;
   bottom: 0;
   background: $ls-overlay; // 暖调遮罩，非纯黑
-  z-index: 9999;
+  z-index: 99999;
   opacity: 0;
   visibility: hidden;
   transition: opacity $uni-transition-base;