Introduce server TTS support and engine selection while keeping
device TTS as the default.
- Add new persistence keys for storing TTS engine and selected
server voice (ttsEngine, ttsServerVoiceId, ttsServerVoiceName).
- Extend TextToSpeechService to support two engines:
TtsEngine.device (FlutterTts) and TtsEngine.server (remote audio).
- Wire in an AudioPlayer and optional ApiService to fetch raw
audio bytes from the server and play them, with event hooks
mapped to existing lifecycle callbacks.
- Implement fallback to device TTS on server errors or empty
responses, and ensure player lifecycle (pause/stop/dispose)
is handled when using server engine.
- Allow engine and preferred voice to be configured before
initialization and updated at runtime via updateSettings.
This enables selecting a server-side voice and using a remote
TTS provider while preserving compatibility with the existing
device TTS implementation.
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