Fix: Identities Group Management - bessere Fehlermeldungen

- add_user_to_group: Exception werfen mit stderr Nachricht
- remove_user_from_group: Exception werfen mit stderr Nachricht
- text=True für subprocess für besseres Error Handling
- Router aktualisiert um Fehlermeldungen an Frontend weiterzugeben
- Benutzer sehen jetzt detaillierte Fehlermeldungen beim Gruppe-Entfernen

Behebt: 'Failed to remove user from group' verschluckt die echte Fehlermeldung

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/services/identities.py

@@ -234,6 +234,7 @@ class IdentitiesManager:
             result = subprocess.run(
                 ["/usr/sbin/usermod", "-aG", groupname, username],
                 capture_output=True,
+                text=True,
                 timeout=10
             )
 
@@ -241,11 +242,12 @@ class IdentitiesManager:
                 logger.info(f"User {username} added to group {groupname}")
                 return True
             else:
-                logger.error(f"Failed to add user to group: {result.stderr.decode()}")
-                return False
+                error_msg = result.stderr.strip() or result.stdout.strip() or "Unknown error"
+                logger.error(f"Failed to add user to group: {error_msg}")
+                raise Exception(f"Failed to add user to group: {error_msg}")
         except Exception as e:
             logger.error(f"Error adding user to group: {e}")
-            return False
+            raise
 
     def remove_user_from_group(self, username: str, groupname: str) -> bool:
         """Remove user from group"""
@@ -253,6 +255,7 @@ class IdentitiesManager:
             result = subprocess.run(
                 ["/usr/sbin/gpasswd", "-d", username, groupname],
                 capture_output=True,
+                text=True,
                 timeout=10
             )
 
@@ -260,11 +263,12 @@ class IdentitiesManager:
                 logger.info(f"User {username} removed from group {groupname}")
                 return True
             else:
-                logger.error(f"Failed to remove user from group: {result.stderr.decode()}")
-                return False
+                error_msg = result.stderr.strip() or result.stdout.strip() or "Unknown error"
+                logger.error(f"Failed to remove user from group: {error_msg}")
+                raise Exception(f"Failed to remove user from group: {error_msg}")
         except Exception as e:
             logger.error(f"Error removing user from group: {e}")
-            return False
+            raise
 
     def change_password(self, username: str, password: str) -> bool:
         """Change user password via chpasswd"""