Track speech state via AVSpeechSynthesizer delegate and fix Stop Speaking validation

- Add _speechSynthesizerIsSpeaking flag updated on didStart/didFinish/didCancel.
- Set AVSpeechSynthesizer delegate in init.
- Update start/stop actions and menu validation to use the new flag.
- Reset flag when there is no text to speak.
- Keeps menu items in sync instead of relying on isSpeaking.