feat: Unbekannte und nicht erlaubte Seiten leiten zum Dashboard um

- src/app/not-found.tsx: jede nicht existierende Route landet auf
  /dashboard statt der Next.js-Standard-404-Seite (der bestehende
  Auth-Guard in (app)/layout.tsx schickt nicht angemeldete User von
  dort automatisch weiter zu /login).
- settings/layout.tsx: fehlt die Berechtigung fuer den
  Einstellungen-Bereich, wird jetzt zu /dashboard umgeleitet statt nur
  einen Hinweistext anzuzeigen (gleiches Redirect-Muster wie der
  bestehende Login-Guard).

Login-nach-Anmeldung-Redirect zu /dashboard und der Auth-Gate in
(app)/layout.tsx bestanden bereits.

ApiCore-Frontend/src/app/(app)/settings/layout.spec.tsx

@@ -0,0 +1,46 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import SettingsLayout from "./layout";
+
+const replaceMock = vi.fn();
+vi.mock("next/navigation", () => ({
+  useRouter: () => ({ replace: replaceMock }),
+}));
+
+let mockUser: { permissions: string[] } | null = null;
+vi.mock("@/lib/auth/auth-context", () => ({
+  useAuth: () => ({ user: mockUser }),
+}));
+
+describe("SettingsLayout", () => {
+  beforeEach(() => {
+    replaceMock.mockReset();
+    mockUser = null;
+  });
+
+  it("renders children when the user has a settings-area permission", () => {
+    mockUser = { permissions: ["users:read"] };
+
+    render(
+      <SettingsLayout>
+        <p>Einstellungen-Inhalt</p>
+      </SettingsLayout>,
+    );
+
+    expect(screen.getByText("Einstellungen-Inhalt")).toBeInTheDocument();
+    expect(replaceMock).not.toHaveBeenCalled();
+  });
+
+  it("redirects to the dashboard when the user lacks every settings-area permission", () => {
+    mockUser = { permissions: ["colonies:read"] };
+
+    render(
+      <SettingsLayout>
+        <p>Einstellungen-Inhalt</p>
+      </SettingsLayout>,
+    );
+
+    expect(screen.queryByText("Einstellungen-Inhalt")).not.toBeInTheDocument();
+    expect(replaceMock).toHaveBeenCalledWith("/dashboard");
+  });
+});

ApiCore-Frontend/src/app/(app)/settings/layout.tsx

@@ -1,6 +1,7 @@
 "use client";
 
-import { ReactNode } from "react";
+import { ReactNode, useEffect } from "react";
+import { useRouter } from "next/navigation";
 import { useAuth } from "@/lib/auth/auth-context";
 
 const SETTINGS_AREA_PERMISSIONS = [
@@ -14,19 +15,20 @@ const SETTINGS_AREA_PERMISSIONS = [
 ];
 
 export default function SettingsLayout({ children }: { children: ReactNode }) {
+  const router = useRouter();
   const { user } = useAuth();
   const hasSettingsAccess = SETTINGS_AREA_PERMISSIONS.some((permission) =>
     user?.permissions.includes(permission),
   );
 
+  useEffect(() => {
     if (!hasSettingsAccess) {
-    return (
-      <main className="p-6">
-        <p className="text-sm text-muted-foreground">
-          Diese Seite ist nur für Administratoren verfügbar.
-        </p>
-      </main>
-    );
+      router.replace("/dashboard");
+    }
+  }, [hasSettingsAccess, router]);
+
+  if (!hasSettingsAccess) {
+    return null;
   }
 
   return <>{children}</>;

ApiCore-Frontend/src/app/not-found.tsx

@@ -0,0 +1,5 @@
+import { redirect } from "next/navigation";
+
+export default function NotFound() {
+  redirect("/dashboard");
+}