fix(onboarding): preserve selected integrations when navigating back

The multi-select grid initializes its own internal Set on mount, so
returning from the configure phase to the select phase lost all
selections. Accept an initialSelection prop and pass the parent's
selectedKinds to restore state on re-mount.

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

@@ -17,15 +17,17 @@ import {
 
 interface IntegrationMultiSelectGridProps {
   onSelectionChange: (kinds: IntegrationKind[]) => void;
+  initialSelection?: IntegrationKind[];
   enableMockIntegration?: boolean;
 }
 
 export const IntegrationMultiSelectGrid = ({
   onSelectionChange,
+  initialSelection = [],
   enableMockIntegration = false,
 }: IntegrationMultiSelectGridProps) => {
   const [search, setSearch] = useState("");
-  const [selectedKinds, setSelectedKinds] = useState<Set<IntegrationKind>>(new Set());
+  const [selectedKinds, setSelectedKinds] = useState<Set<IntegrationKind>>(() => new Set(initialSelection));
   const t = useI18n();
 
   const toggleKind = useCallback(