fix(ui): add custom xxl modal size to prevent overflow on small screens

Hardcoded size: 1200 could overflow on narrow viewports. Add a custom
"xxl" size (75rem) to the Mantine Modal theme via a vars resolver.
Mantine caps modal width at 100vw so it naturally adapts on mobile.
Use "xxl" in item-select, app-select, and integration-select modals.

apps/nextjs/src/components/board/items/item-select-modal.tsx

@@ -70,7 +70,7 @@ export const ItemSelectModal = createModal<void>(({ actions }) => {
   );
 }).withOptions({
   defaultTitle: (t) => t("item.create.title"),
-  size: 1200,
+  size: "xxl",
 });
 
 const WidgetItem = ({

apps/nextjs/src/components/integration/integration-select-modal.tsx

@@ -52,5 +52,5 @@ export const IntegrationSelectModal = createModal<IntegrationSelectModalProps>((
   return <IntegrationSelectGrid onSelect={handleSelect} enableMockIntegration={innerProps.enableMockIntegration} />;
 }).withOptions({
   defaultTitle: (t) => t("integration.action.create"),
-  size: 1200,
+  size: "xxl",
 });