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)

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
         )

gui/user_selection_dialog.py

@@ -0,0 +1,157 @@
+"""
+User Selection Dialog - Allows Admin to choose which user identity to use
+"""
+import tkinter as tk
+from tkinter import ttk, messagebox
+from typing import Optional, Dict, Any, List
+
+
+class UserSelectionDialog:
+    """
+    Modal dialog for Admin users to select which user identity to use
+
+    Usage:
+        dialog = UserSelectionDialog(parent, users_list, current_username)
+        selected_user = dialog.get_selected_user()
+        if selected_user:
+            # Use the selected user info
+    """
+
+    def __init__(self, parent, users: List[Dict[str, Any]], current_username: str):
+        """
+        Initialize the user selection dialog
+
+        Args:
+            parent: The parent Tkinter window
+            users: List of user dictionaries [{id, username, user_type}]
+            current_username: The currently logged-in Admin's username
+        """
+        self.parent = parent
+        self.users = users
+        self.current_username = current_username
+        self.selected_user = None
+        self.dialog = None
+        self.selected_var = None
+
+        self._create_dialog()
+        self.parent.wait_window(self.dialog)
+
+    def _create_dialog(self):
+        """Create the user selection dialog UI"""
+        self.dialog = tk.Toplevel(self.parent)
+        self.dialog.title("选择用户身份")
+        self.dialog.geometry("450x400")
+        self.dialog.resizable(False, False)
+
+        # Center on screen (not parent)
+        self.dialog.transient(self.parent)
+        self.dialog.grab_set()
+
+        screen_width = self.dialog.winfo_screenwidth()
+        screen_height = self.dialog.winfo_screenheight()
+        dialog_width = 450
+        dialog_height = 400
+        x = (screen_width - dialog_width) // 2
+        y = (screen_height - dialog_height) // 2
+        self.dialog.geometry(f"{dialog_width}x{dialog_height}+{x}+{y}")
+
+        self._create_widgets()
+
+    def _create_widgets(self):
+        """Create dialog widgets"""
+        main_frame = ttk.Frame(self.dialog, padding="20")
+        main_frame.pack(fill=tk.BOTH, expand=True)
+
+        # Title
+        title_label = ttk.Label(
+            main_frame,
+            text="请选择要使用的用户身份",
+            font=('', 14, 'bold')
+        )
+        title_label.pack(pady=(0, 20))
+
+        # User list with scrollbar
+        list_frame = ttk.Frame(main_frame)
+        list_frame.pack(fill=tk.BOTH, expand=True)
+
+        scrollbar = ttk.Scrollbar(list_frame)
+        scrollbar.pack(side=tk.RIGHT, fill=tk.Y)
+
+        self.selected_var = tk.StringVar()
+
+        # Sort users: current user first, then by username
+        sorted_users = sorted(
+            self.users,
+            key=lambda u: (0 if u['username'] == self.current_username else 1, u['username'])
+        )
+
+        for user in sorted_users:
+            username = user['username']
+            user_type = user['user_type']
+            is_current = username == self.current_username
+
+            # Mark current user
+            display_text = f"{username} ({user_type})"
+            if is_current:
+                display_text += " [当前]"
+
+            rb = ttk.Radiobutton(
+                list_frame,
+                text=display_text,
+                variable=self.selected_var,
+                value=username
+            )
+            rb.pack(anchor=tk.W, pady=3, padx=5)
+
+        # Select current user by default
+        self.selected_var.set(self.current_username)
+
+        # Buttons
+        button_frame = ttk.Frame(main_frame)
+        button_frame.pack(pady=(20, 0))
+
+        confirm_btn = ttk.Button(
+            button_frame,
+            text="确认",
+            command=self._on_confirm,
+            width=10
+        )
+        confirm_btn.pack(side=tk.LEFT, padx=5)
+
+        cancel_btn = ttk.Button(
+            button_frame,
+            text="取消",
+            command=self._on_cancel,
+            width=10
+        )
+        cancel_btn.pack(side=tk.LEFT, padx=5)
+
+    def _on_confirm(self):
+        """Handle confirm button click"""
+        selected_username = self.selected_var.get()
+
+        if not selected_username:
+            messagebox.showwarning("未选择", "请选择一个用户")
+            return
+
+        # Find the selected user
+        for user in self.users:
+            if user['username'] == selected_username:
+                self.selected_user = user
+                break
+
+        self.dialog.destroy()
+
+    def _on_cancel(self):
+        """Handle cancel button click"""
+        self.selected_user = None
+        self.dialog.destroy()
+
+    def get_selected_user(self) -> Optional[Dict[str, Any]]:
+        """
+        Get the selected user info
+
+        Returns:
+            User dict {id, username, user_type} if confirmed, None if cancelled
+        """
+        return self.selected_user