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/repository/user_repository.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"strings"
 
+	"github.com/lib/pq"
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
 )
@@ -156,11 +157,12 @@ func (r *userRepository) CountByCompanyCodes(ctx context.Context, codes []string
 	}
 	var results []result
 
-	// Search by company_code directly. Normalize inputs using LOWER for robust matching.
-	err := r.db.WithContext(ctx).Model(&domain.User{}).
-		Select("LOWER(company_code) as company_code, count(*) as count").
-		Where("LOWER(company_code) IN ?", lowerStrings(codes)).
-		Group("LOWER(company_code)").
+	// Search by company_codes array using unnest and overlap. 
+	// This ensures users with multiple memberships are counted in each tenant they belong to.
+	err := r.db.WithContext(ctx).Table("users").
+		Select("unnest(company_codes) as company_code, count(*) as count").
+		Where("company_codes && ?", pq.Array(lowerStrings(codes))).
+		Group("company_code").
 		Scan(&results).Error
 	if err != nil {
 		return nil, err
@@ -168,7 +170,7 @@ func (r *userRepository) CountByCompanyCodes(ctx context.Context, codes []string
 
 	counts := make(map[string]int64)
 	for _, res := range results {
-		counts[res.CompanyCode] = res.Count
+		counts[strings.ToLower(res.CompanyCode)] = res.Count
 	}
 
 	// Ensure all requested codes are present in results (even if count is 0)
@@ -196,15 +198,15 @@ func (r *userRepository) List(ctx context.Context, offset, limit int, search str
 	db := r.db.WithContext(ctx).Model(&domain.User{})
 
 	if companyCode != "" {
-		// [Matrix Fix] Match users either by their primary company code OR by the slug of the department they are attached to
+		// [Matrix Fix] Match users either by their primary company code OR by being in the company_codes array OR by tenant slug
 		db = db.Joins("LEFT JOIN tenants ON users.tenant_id = tenants.id").
-			Where("users.company_code = ? OR tenants.slug = ?", companyCode, companyCode)
+			Where("users.company_code = ? OR ? = ANY(users.company_codes) OR tenants.slug = ?", companyCode, companyCode, companyCode)
 	}
 
 	if search != "" {
 		searchTerm := "%" + search + "%"
-		db = db.Where("(users.email LIKE ? OR users.name LIKE ? OR users.company_code LIKE ? OR users.metadata::text LIKE ?)",
-			searchTerm, searchTerm, searchTerm, searchTerm)
+		db = db.Where("(users.email LIKE ? OR users.name LIKE ? OR users.company_code LIKE ? OR ? = ANY(users.company_codes) OR users.metadata::text LIKE ?)",
+			searchTerm, searchTerm, searchTerm, search, searchTerm)
 	}
 
 	if err := db.Count(&total).Error; err != nil {