feat(FN-887): widen model menu on mobile viewports

- Widen quick-entry Models menu to fill viewport on mobile (≤640px) with side padding
- Keep menu viewport-clamped to prevent horizontal crowding on small screens
- Update dashboard README with mobile model menu documentation

packages/dashboard/app/styles.css

@@ -11685,6 +11685,8 @@ html .column.drag-over * {
   max-width: 360px;
 }
 
+/* Mobile: portaled model menu uses wider viewport-clamped width set via inline styles */
+
 .model-menu-items {
   display: flex;
   flex-direction: column;
@@ -11783,6 +11785,11 @@ html .column.drag-over * {
     max-width: unset;
     width: calc(100vw - 32px);
   }
+
+  /* Portaled variant on mobile: remove max-width constraint so JS-driven width takes full effect */
+  .model-nested-menu--portal {
+    max-width: none;
+  }
 }
 
 /* Responsive layout for quick entry controls */