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}`;
+    },
+};

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",
+    },
+];

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,
+    },
+];