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/lib/navigation/sidebar-items.ts

@@ -0,0 +1,38 @@
+import {AppWindowIcon, Home, LayoutDashboard} from "lucide-react";
+import {MenuItem, SubMenuItem} from "@/types/navigation/sidebar";
+
+export const rheinItems: MenuItem[] = [
+    {
+        title: "Dashboard",
+        url: "/",
+        icon: Home,
+    },
+    {
+        title: "Apps",
+        url: "/apps",
+        icon: AppWindowIcon,
+    },
+];
+
+export const customerItems: MenuItem[] = [
+    {
+        title: "Kundenübersicht",
+        url: "/customers",
+        icon: LayoutDashboard,
+    },
+];
+
+export const kanzleiItems: SubMenuItem[] = [
+    {
+        title: "Steuer",
+        url: "/demo/kanzlei/steuer",
+    },
+    {
+        title: "Rechtsanwalt",
+        url: "/demo/kanzlei/rechtsanwalt",
+    },
+    {
+        title: "Bilanzbuchhalter",
+        url: "/demo/kanzlei/bilanzbuchhalter",
+    },
+];