Refactor navigation structure and API routes

- Centralize user menu, sidebar items, and breadcrumb logic.
- Map consistent API endpoints in `customerRoutes`.
- Replace inline route definitions with reusable constants.
- Refactor auth configuration file location.
- Improve `<Link>` usage to replace static `<a>` elements.
- Adjust sidebar and dropdown components to use dynamic navigation configurations.

internal_frontend/app/layout.tsx

@@ -8,7 +8,7 @@ import {Separator} from "@/components/ui/separator";
 import {DynamicBreadcrumb} from "@/components/dynamic-breadcrumb";
 import {getServerSession} from "next-auth";
 import LoginScreen from "@/components/login-screen";
-import {authOptions} from "@/lib/auth/authOptions";
+import {authOptions} from "@/lib/api/auth/authOptions";
 
 export const metadata: Metadata = {
     title: "Internal | Rhein Software",