POS Customer Creation Tasks

[x] Backend Bug Fix: Eager-load User model on PosController::posCustomer()
[x] Backend Extension: Add PosController::storeCustomer to handle first_name, mobile, email
[x] Backend Routing: Register Route::post('/pos/customer/create')
[x] Frontend Data: Inject createCustomer method into pos_service_provider.dart
[x] Frontend State: Expand PosControllerNotifier to coordinate posCustomersProvider invalidation
[x] Frontend View: Restructure Customer Form array in pos_view.dart with dialog handlers