v1.8.12: fix Ctrl+Z undo — manual implementation for tk.Entry

tk.Entry has no built-in undo (unlike tk.Text), so _entry.config(undo=True)
crashed. Replaced with custom entry_undo.py that tracks history manually
and binds Ctrl+Z/Ctrl+Y (+ Russian layout support).

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

gui/tabs/totp_tab.py

@@ -6,6 +6,7 @@ Live countdown, one-click copy, per-server secrets.
 import threading
 import customtkinter as ctk
 from core.i18n import t
+from gui.widgets.entry_undo import enable_undo
 
 
 class TOTPTab(ctk.CTkFrame):
@@ -106,8 +107,7 @@ class TOTPTab(ctk.CTkFrame):
             font=ctk.CTkFont(family="Consolas", size=12)
         )
         self.secret_entry.pack(side="left", fill="x", expand=True, padx=(0, 5))
-        # Enable undo functionality
-        self.secret_entry._entry.config(undo=True)
+        enable_undo(self.secret_entry)
 
         self.show_secret_btn = ctk.CTkButton(
             entry_row, text=t("show"), width=70,