feat(tts): server-backed TTS engine selection

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.

pubspec.yaml

@@ -47,6 +47,7 @@ dependencies:
   record: ^6.1.1
   stts: ^1.2.5
   flutter_tts: ^4.2.3
+  audioplayers: ^5.2.1
   image_picker: ^1.2.0
   file_picker: ^10.3.3
   path_provider: ^2.1.4