fix(ui): select dropdown obscured by header in landscape mode

Fixes #14073

This change adds a new CSS variable --z-select-menu: 35 to the global
z-index system and applies it to ReactSelect menu styles. This ensures
Select dropdown menus appear above the AppHeader when opening upward
in landscape mobile mode.

The z-index value of 35 places the dropdown between --z-modal (30)
and --z-status (40), following the established z-index hierarchy.

Author: ahmed-abdat

packages/ui/src/elements/ReactSelect/index.scss

@@ -36,7 +36,7 @@
     }
 
     .rs__menu {
-      z-index: 4;
+      z-index: var(--z-select-menu);
       border-radius: 0;
       @include shadow-lg;
       background: var(--theme-input-bg);

packages/ui/src/scss/app.scss

@@ -34,6 +34,7 @@
     --z-popup: 10;
     --z-nav: 20;
     --z-modal: 30;
+    --z-select-menu: 35;
     --z-status: 40;
 
     --accessibility-outline: 2px solid var(--theme-text);