feat: Add Text-to-Speech settings and customization options

- Introduced new preference keys for TTS settings: voice, speech rate, pitch, and volume.
- Updated SettingsService to handle TTS settings and persist them.
- Enhanced AppSettings to include TTS-related properties.
- Implemented TTS settings UI in AppCustomizationPage, allowing users to select voice and adjust speech parameters.
- Added localization support for TTS settings in multiple languages.

lib/l10n/app_zh.arb

@@ -309,6 +309,19 @@
   "chatSettings": "对话",
   "sendOnEnter": "回车发送",
   "sendOnEnterDescription": "回车发送（软键盘）。Cmd/Ctrl+Enter 也可用",
+  "ttsSettings": "文本转语音",
+  "ttsVoice": "语音",
+  "ttsSpeechRate": "语速",
+  "ttsPitch": "音调",
+  "ttsVolume": "音量",
+  "ttsPreview": "预览语音",
+  "ttsSystemDefault": "系统默认",
+  "ttsSelectVoice": "选择语音",
+  "ttsPreviewText": "这是所选语音的预览。",
+  "ttsNoVoicesAvailable": "没有可用的语音",
+  "ttsVoicesForLanguage": "{language} 语音",
+  "ttsOtherVoices": "其他语言",
+  "error": "错误",
   "display": "显示",
   "realtime": "实时",
   "transportMode": "传输模式",