fix: editable alias in server dialog + Ctrl+Z undo for all input fields

- Alias field no longer disabled when editing server profile
- Duplicate alias check on rename, session pool migration
- Enable undo (Ctrl+Z) on all CTkEntry widgets across the project

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

core/i18n.py

@@ -90,6 +90,7 @@ _EN = {
     "show": "Show",
     "hide": "Hide",
     "alias_required": "Alias is required",
+    "alias_exists": "Alias '{alias}' already exists",
     "ip_required": "IP is required",
     "port_must_be_number": "Port must be a number",
     "error_prefix": "Error: {msg}",
@@ -355,6 +356,7 @@ _RU = {
     "show": "Показать",
     "hide": "Скрыть",
     "alias_required": "Алиас обязателен",
+    "alias_exists": "Алиас '{alias}' уже существует",
     "ip_required": "IP обязателен",
     "port_must_be_number": "Порт должен быть числом",
     "error_prefix": "Ошибка: {msg}",
@@ -620,6 +622,7 @@ _ZH = {
     "show": "显示",
     "hide": "隐藏",
     "alias_required": "别名不能为空",
+    "alias_exists": "别名 '{alias}' 已存在",
     "ip_required": "IP不能为空",
     "port_must_be_number": "端口必须是数字",
     "error_prefix": "错误：{msg}",