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/customerRoutes.ts

@@ -1,4 +1,5 @@
 export const customerRoutes = {
     create: "/customers",
     validate: "/customers/validate",
+    getById: (id: string) => `/customers/${id}`,
 };