feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

userfront/lib/core/services/auth_proxy_service.dart

@@ -964,7 +964,6 @@ class AuthProxyService {
 
   static Future<void> signup({
     required String email,
-    String? loginId,
     required String password,
     required String name,
     required String phone,
@@ -980,12 +979,11 @@ class AuthProxyService {
       headers: {'Content-Type': 'application/json'},
       body: jsonEncode({
         'email': email,
-        if (loginId != null && loginId.isNotEmpty) 'loginId': loginId,
         'password': password,
         'name': name,
         'phone': phone,
         'affiliationType': affiliationType,
-        'companyCode': ?companyCode,
+        'companyCode': companyCode,
         'department': department,
         'termsAccepted': termsAccepted,
       }),