fix: Change undo/redo keys to Ctrl+U/Ctrl+R

Ctrl+Z is the Unix suspend signal (SIGTSTP) which suspends the process
before urwid can intercept it. Changed to:
- Ctrl+U for undo
- Ctrl+R for redo

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/smarttools/ui_urwid.py

@@ -345,9 +345,11 @@ class UndoableEdit(urwid.Edit):
     """A multiline Edit with undo/redo support.
 
     Features:
-    - Undo with Ctrl+Z (up to 50 states)
+    - Undo with Ctrl+U (up to 50 states)
-    - Redo with Ctrl+Y or Ctrl+Shift+Z
+    - Redo with Ctrl+R
     - Tab passes through for focus cycling
+
+    Note: Ctrl+Z cannot be used as it's the Unix suspend signal (SIGTSTP).
     """
 
     MAX_UNDO = 50  # Maximum undo history size
@@ -368,13 +370,13 @@ class UndoableEdit(urwid.Edit):
             self._save_undo_state(self._last_saved_text, self.edit_pos)
             self._last_saved_text = current_text
 
-        # Handle undo (Ctrl+Z)
+        # Handle undo (Ctrl+U) - can't use Ctrl+Z as it's Unix suspend signal
-        if key == 'ctrl z':
+        if key == 'ctrl u':
             self._undo()
             return None
 
-        # Handle redo (Ctrl+Y or Ctrl+Shift+Z)
+        # Handle redo (Ctrl+R)
-        if key in ('ctrl y', 'ctrl shift z'):
+        if key == 'ctrl r':
             self._redo()
             return None
 
@@ -1685,7 +1687,7 @@ class SmartToolsUI:
         default_file = existing.code_file if existing and existing.code_file else f"{default_output_var}.py"
         file_edit = urwid.Edit(('label', "File: "), default_file)
 
-        # Multiline code editor with undo/redo (Ctrl+Z / Ctrl+Y)
+        # Multiline code editor with undo/redo (Ctrl+U / Ctrl+R)
         default_code = existing.code if existing else f"{default_output_var} = input.upper()"
         code_edit = UndoableEdit(
             edit_text=default_code,