Preserve configured TTS fallback for chat playback

Leave chat playback provider unset so TextToSpeechService can distinguish configured/default playback from explicit provider requests and apply the new Windows fallback when the configured provider is unavailable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: shanselman

src/OpenClaw.Tray.WinUI/Chat/OpenClawChatCoordinator.cs

@@ -138,7 +138,9 @@ private async Task SpeakConfiguredTextAsync(string text, bool muteVoiceCapture,
             var speakArgs = new TtsSpeakArgs
             {
                 Text = speakText,
-                Provider = _settings.TtsProvider ?? TtsCapability.PiperProvider,
+                // Leave provider unset so TextToSpeechService treats this as
+                // configured/default playback and can fall back when needed.
+                Provider = null,
                 Interrupt = true
             };