v1.8.0: file manager improvements

- SFTP cleanup on app close and language switch
- Windows drive selector in local panel
- Browse and Refresh buttons for local panel
- Recursive upload/download/delete of folders
- Drag-and-drop between local and remote panels
- Sudo mode toggle for privileged file operations
- New i18n keys for EN/RU/ZH

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/app.py

@@ -171,8 +171,9 @@ class App(ctk.CTk):
         # Use provided key or default to first tab
         current_key = restore_tab_key or self._tab_keys[0]
 
-        # Disconnect terminal before destroying tabs
+        # Disconnect terminal and SFTP before destroying tabs
         self.terminal_tab._disconnect()
+        self.files_tab._disconnect_sftp()
 
         # Detach tab contents
         self.terminal_tab.pack_forget()
@@ -227,5 +228,6 @@ class App(ctk.CTk):
 
     def _on_close(self):
         self.terminal_tab._disconnect()
+        self.files_tab._disconnect_sftp()
         self.checker.stop()
         self.destroy()