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/user-menu-items.ts

@@ -0,0 +1,20 @@
+import {LogOut, Settings} from "lucide-react";
+
+export interface UserMenuItem {
+    title: string;
+    url: string;
+    icon?: typeof Settings;
+}
+
+export const userMenuItems: UserMenuItem[] = [
+    {
+        title: "Settings",
+        url: "/settings",
+        icon: Settings,
+    },
+    {
+        title: "Ausloggen",
+        url: "/api/auth/signout",
+        icon: LogOut,
+    },
+];