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/api/customers/[id]/route.ts

@@ -1,9 +1,10 @@
 import {NextRequest, NextResponse} from "next/server";
 import {serverCall} from "@/lib/api/serverCall";
+import {customerRoutes} from "@/app/api/customers/customerRoutes";
 
 export async function GET(request: NextRequest) {
     const id = request.url.split('/').pop();
-    const response = await serverCall(`/customers/${id}`, "GET");
+    const response = await serverCall(customerRoutes.getById(id!), "GET");
 
     if (!response.ok) {
         return NextResponse.json({error: "Customer not found"}, {status: 404});