fix(FN-667): increase model dropdown z-index to fix overlay stacking

- Increase z-index from 100 to 500 for .model-selector-dropdown

- Fix dropdown appearing behind modal overlays and other UI elements

packages/dashboard/app/styles.css

@@ -6077,7 +6077,7 @@ body {
   border: 1px solid var(--border);
   border-radius: var(--radius);
   box-shadow: var(--shadow);
-  z-index: 100;
+  z-index: 500;
   max-height: 320px;
   display: flex;
   flex-direction: column;