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.lock

@@ -65,6 +65,62 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.13.0"
+  audioplayers:
+    dependency: "direct main"
+    description:
+      name: audioplayers
+      sha256: c05c6147124cd63e725e861335a8b4d57300b80e6e92cea7c145c739223bbaef
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.2.1"
+  audioplayers_android:
+    dependency: transitive
+    description:
+      name: audioplayers_android
+      sha256: b00e1a0e11365d88576320ec2d8c192bc21f1afb6c0e5995d1c57ae63156acb5
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.0.3"
+  audioplayers_darwin:
+    dependency: transitive
+    description:
+      name: audioplayers_darwin
+      sha256: "3034e99a6df8d101da0f5082dcca0a2a99db62ab1d4ddb3277bed3f6f81afe08"
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.0.2"
+  audioplayers_linux:
+    dependency: transitive
+    description:
+      name: audioplayers_linux
+      sha256: "60787e73fefc4d2e0b9c02c69885402177e818e4e27ef087074cf27c02246c9e"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.0"
+  audioplayers_platform_interface:
+    dependency: transitive
+    description:
+      name: audioplayers_platform_interface
+      sha256: "365c547f1bb9e77d94dd1687903a668d8f7ac3409e48e6e6a3668a1ac2982adb"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.1.0"
+  audioplayers_web:
+    dependency: transitive
+    description:
+      name: audioplayers_web
+      sha256: "22cd0173e54d92bd9b2c80b1204eb1eb159ece87475ab58c9788a70ec43c2a62"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.0"
+  audioplayers_windows:
+    dependency: transitive
+    description:
+      name: audioplayers_windows
+      sha256: "9536812c9103563644ada2ef45ae523806b0745f7a78e89d1b5fb1951de90e1a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.0"
   boolean_selector:
     dependency: transitive
     description: