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/sidebar.py

@@ -5,6 +5,7 @@ Sidebar — server list with search, add/edit/delete buttons.
 import customtkinter as ctk
 from core.i18n import t
 from gui.widgets.status_badge import StatusBadge
+from gui.widgets.entry_undo import enable_undo
 
 
 class Sidebar(ctk.CTkFrame):
@@ -29,8 +30,7 @@ class Sidebar(ctk.CTkFrame):
         self.search_var.trace_add("write", lambda *_: self._refresh_list())
         self.search_entry = ctk.CTkEntry(self, placeholder_text=t("search"), textvariable=self.search_var)
         self.search_entry.pack(fill="x", padx=10, pady=(5, 10))
-        # Enable undo functionality
-        self.search_entry._entry.config(undo=True)
+        enable_undo(self.search_entry)
 
         # Server list
         self.list_frame = ctk.CTkScrollableFrame(self, fg_color="transparent")