fix(init): improve integrations step UX

- Replace plain Base URL input with a prominent Alert callout so users
  don't overlook it
- Add disableScroll prop to SelectGridLayout and use it in onboarding
  so integrations expand the page instead of scrolling
- Fix Collapse `in` → `expanded` prop for Mantine v9 migration in both
  the URL helpers panel and the Docker discovery panel

apps/nextjs/src/components/integration/integration-multi-select-grid.tsx

@@ -51,6 +51,7 @@ export const IntegrationMultiSelectGrid = ({
       search={search}
       onSearchChange={setSearch}
       placeholder={`${t("integration.page.list.search")}...`}
+      disableScroll={onboarding}
     >
       {filtered.map((integration) => {
         const isSelected = selectedKinds.has(integration.kind);