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>

gui/main_window.py

@@ -102,8 +102,17 @@ class MainWindow:
 
         # 用户信息显示
         user_type_display = "管理员" if self.session_manager.is_admin() else "用户"
+        original_admin = self.session_manager.get_original_admin()
+
+        if original_admin:
+            # Admin以其他用户身份操作
+            user_info_text = f"当前用户: {self.session_manager.get_username()} ({user_type_display}) - 以 {original_admin['username']} 身份登录"
+        else:
+            # 正常登录
+            user_info_text = f"当前用户: {self.session_manager.get_username()} ({user_type_display})"
+
         self.user_info_var = tk.StringVar()
-        self.user_info_var.set(f"当前用户: {self.session_manager.get_username()} ({user_type_display})")
+        self.user_info_var.set(user_info_text)
         user_label = ttk.Label(
             self.status_bar, textvariable=self.user_info_var, anchor=tk.E
         )