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/breadcrumb-map.ts

@@ -0,0 +1,19 @@
+import {customerRoutes} from "@/app/api/customers/customerRoutes";
+
+export const breadcrumbMap: Record<string, string> = {
+    'dashboard': 'Dashboard',
+    'settings': 'Settings',
+    'demo': 'Demo',
+    'users': 'User Management',
+    'customers': 'Kundenübersicht',
+};
+
+export const breadcrumbResolvers: Record<string, (id: string) => Promise<string>> = {
+    "customers": async (id: string) => {
+        const res = await fetch(`/api${customerRoutes.getById(id)}`, {cache: "no-store"});
+        const customer = await res.json();
+        if (customer.companyName) return `Firma: ${customer.companyName}`;
+        if (customer.name) return `Name: ${customer.name}`;
+        return `ID: ${id}`;
+    },
+};