feat: implement multi-tenant member management and UI improvements

- Add multi-tenant support (isAddTenant, isRemoveTenant) to backend UpdateUser API.
- Update UserRepository to support searching in company_codes array.
- Implement table sorting and align search bar layout in adminfront.
- Add 'Assign Existing Member' and 'Exclude from Organization' features to TenantUsersPage.
- Auto-populate tenantSlug in UserCreatePage via query parameters.
- Add necessary localization keys for new UI elements.

Resolves #644, #639, #642, #641

backend/internal/domain/user.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/lib/pq"
 	"gorm.io/gorm"
 )
 
@@ -54,6 +55,7 @@ type User struct {
 	Role            string         `gorm:"column:role;default:'user';not null" json:"role"` // super_admin, tenant_admin, rp_admin, user
 	AffiliationType string         `gorm:"column:affiliation_type" json:"affiliationType"`
 	CompanyCode     string         `gorm:"column:company_code;index" json:"companyCode"`
+	CompanyCodes    pq.StringArray `gorm:"column:company_codes;type:text[]" json:"companyCodes"`
 	TenantID        *string        `gorm:"column:tenant_id;type:uuid;index" json:"tenantId,omitempty"`
 	Tenant          *Tenant        `gorm:"foreignKey:TenantID" json:"tenant,omitempty"`
 	RelyingPartyID  *string        `gorm:"column:relying_party_id;type:uuid;index" json:"relyingPartyId,omitempty"` // RP Admin용