feat: add Admin user selection dialog (管理员用户选择对话框)

When an Admin user logs in, display a centered dialog allowing them to
select which user identity to use for the session. The Admin adopts the
selected user's permissions and operates as that user throughout the session.

Changes:
- Add UserSelectionDialog class in gui/user_selection_dialog.py
- Add SessionManager.switch_user() method to switch user identity
- Add SessionManager.get_original_admin() method to retrieve original admin
- Update MainWindow status bar to show when Admin operates as another user
- Display format: "当前用户: {username} ({type}) - 以 {admin_username} 身份登录"

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

auth/session_manager.py

@@ -1,7 +1,7 @@
 """
 Session Manager - Singleton pattern for managing authenticated user session
 """
-from typing import Optional
+from typing import Optional, Dict, Any
 
 
 class SessionManager:
@@ -150,3 +150,38 @@ class SessionManager:
             Dict with username and user_type if authenticated, None otherwise
         """
         return self._current_user
+
+    def switch_user(self, user_info: Dict[str, Any]) -> bool:
+        """
+        Switch to a different user (Admin only feature)
+
+        This allows an Admin user to operate as a different user
+        with that user's permissions.
+
+        Args:
+            user_info: Dict with {id, username, user_type}
+
+        Returns:
+            True if switch successful
+        """
+        if not self._current_user:
+            return False
+
+        # Store original admin user for reference
+        if not hasattr(self, '_original_admin_user'):
+            self._original_admin_user = self._current_user.copy()
+
+        self._current_user = {
+            'username': user_info['username'],
+            'user_type': user_info['user_type']
+        }
+        return True
+
+    def get_original_admin(self) -> Optional[Dict[str, Any]]:
+        """
+        Get the original Admin user before any user switch
+
+        Returns:
+            Original admin user dict if a switch occurred, None otherwise
+        """
+        return getattr(self, '_original_admin_user', None)