refactor(integrations): make onSuccess required and simplify new page

- Make onSuccess a required prop on NewIntegrationForm so the consumer
  always controls post-creation behavior. Remove internal router.push.
- Add IntegrationNewFormWrapper client component to handle navigation
  for the server-rendered manage page.
- Replace IntegrationPickerPage with a simple "back to overview" button
  when an invalid integration kind is provided. Delete unused
  integration-picker-page.tsx.

apps/nextjs/src/components/integration/integration-picker-page.tsx

@@ -1,25 +0,0 @@
-"use client";
-
-import { useCallback } from "react";
-import { useRouter } from "next/navigation";
-
-import type { IntegrationKind } from "@homarr/definitions";
-
-import { IntegrationSelectGrid } from "./integration-select-grid";
-
-interface IntegrationPickerPageProps {
-  enableMockIntegration: boolean;
-}
-
-export const IntegrationPickerPage = ({ enableMockIntegration }: IntegrationPickerPageProps) => {
-  const router = useRouter();
-
-  const handleSelect = useCallback(
-    (kind: IntegrationKind) => {
-      router.push(`/manage/integrations/new?kind=${kind}`);
-    },
-    [router],
-  );
-
-  return <IntegrationSelectGrid onSelect={handleSelect} enableMockIntegration={enableMockIntegration} />;
-};