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_de.arb

@@ -316,6 +316,19 @@
   "chatSettings": "Chat",
   "sendOnEnter": "Mit Enter senden",
   "sendOnEnterDescription": "Enter sendet (Soft-Tastatur). Cmd/Ctrl+Enter ebenfalls verfügbar",
+  "ttsSettings": "Text zu Sprache",
+  "ttsVoice": "Stimme",
+  "ttsSpeechRate": "Sprechgeschwindigkeit",
+  "ttsPitch": "Tonhöhe",
+  "ttsVolume": "Lautstärke",
+  "ttsPreview": "Stimme vorschau",
+  "ttsSystemDefault": "Systemstandard",
+  "ttsSelectVoice": "Stimme auswählen",
+  "ttsPreviewText": "Dies ist eine Vorschau der ausgewählten Stimme.",
+  "ttsNoVoicesAvailable": "Keine Stimmen verfügbar",
+  "ttsVoicesForLanguage": "{language}-Stimmen",
+  "ttsOtherVoices": "Andere Sprachen",
+  "error": "Fehler",
   "display": "Anzeige",
   "realtime": "Echtzeit",
   "transportMode": "Transportmodus",