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.

ios/Podfile.lock

@@ -1,4 +1,6 @@
 PODS:
+  - audioplayers_darwin (0.0.1):
+    - Flutter
   - connectivity_plus (0.0.1):
     - Flutter
   - DKImagePickerController/Core (4.3.9):
@@ -84,6 +86,7 @@ PODS:
     - FlutterMacOS
 
 DEPENDENCIES:
+  - audioplayers_darwin (from `.symlinks/plugins/audioplayers_darwin/ios`)
   - connectivity_plus (from `.symlinks/plugins/connectivity_plus/ios`)
   - file_picker (from `.symlinks/plugins/file_picker/ios`)
   - Flutter (from `Flutter`)
@@ -113,6 +116,8 @@ SPEC REPOS:
     - SwiftyGif
 
 EXTERNAL SOURCES:
+  audioplayers_darwin:
+    :path: ".symlinks/plugins/audioplayers_darwin/ios"
   connectivity_plus:
     :path: ".symlinks/plugins/connectivity_plus/ios"
   file_picker:
@@ -155,6 +160,7 @@ EXTERNAL SOURCES:
     :path: ".symlinks/plugins/webview_flutter_wkwebview/darwin"
 
 SPEC CHECKSUMS:
+  audioplayers_darwin: ccf9c770ee768abb07e26d90af093f7bab1c12ab
   connectivity_plus: cb623214f4e1f6ef8fe7403d580fdad517d2f7dd
   DKImagePickerController: 946cec48c7873164274ecc4624d19e3da4c1ef3c
   DKPhotoGallery: b3834fecb755ee09a593d7c9e389d8b5d6deed60