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/route.ts

@@ -1,14 +1,15 @@
 import {NextRequest, NextResponse} from "next/server";
 import {serverCall} from "@/lib/api/serverCall";
+import {customerRoutes} from "@/app/api/customers/customerRoutes";
 
 export async function GET() {
-    const data = await serverCall("/customers", "GET");
+    const data = await serverCall(customerRoutes.create, "GET");
     const customers = await data.json();
     return NextResponse.json(customers);
 }
 
 export async function POST(req: NextRequest) {
     const body = await req.json()
-    const result = await serverCall("/customers", "POST", body);
+    const result = await serverCall(customerRoutes.create, "POST", body);
     return NextResponse.json(result.json());
 }