Refactor Customer Entity and Forms to Use 'Companies' Field

- Updated the Customer entity to replace 'CompanyIDs' with 'Companies' for improved clarity and consistency.
- Modified CreateCustomerForm and UpdateCustomerForm to reflect the change from 'CompanyIDs' to 'Companies', ensuring proper validation and structured responses.
- Adjusted service methods to handle the new 'Companies' field, including registration and company assignment logic, enhancing the overall data model and API usability.

internal/customer/entity.go

@@ -44,7 +44,7 @@ type Customer struct {
 	Type        CustomerType   `bson:"type"`
 	Role        CustomerRole   `bson:"role"`
 	Phone       *string        `bson:"phone"`
-	CompanyIDs  []string       `bson:"company_ids"`
+	Companies   []string       `bson:"companies"`
 	DeviceToken []string       `bson:"device_token"`
 	LastLoginAt *int64         `bson:"last_login_at"`
 	UpdatedAt   int64          `bson:"updated_at"`