Add Undo/Redo (Ctrl/Cmd+Z, Ctrl/Cmd+Y)

[Mandip77]

Summary

Implement Undo/Redo using JavaFX TextArea built-ins.

Tasks

• Add "Undo" and "Redo" menu items + accelerators.
• Wire to txtarea.undo() and txtarea.redo().
• Disable the menu items if not available (optional).

Acceptance Criteria

• Keyboard shortcuts work on Win/Linux/Mac.
• Menu actions undo/redo reliably.

Hints

• FXML: add items under Edit menu.
• Controller: simple handlers calling txtarea.undo()/redo().