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.

lib/core/providers/app_providers.dart

@@ -1830,7 +1830,11 @@ Future<List<String>> availableVoices(Ref ref) async {
   if (api == null) return [];
 
   try {
-    return await api.getAvailableVoices();
+    final voices = await api.getAvailableServerVoices();
+    return voices
+        .map((v) => (v['name'] ?? v['id'] ?? '').toString())
+        .where((s) => s.isNotEmpty)
+        .toList();
   } catch (e) {
     DebugLogger.error('voices-failed', scope: 'voices', error: e);
     return [];