fix: harden custom TTS URL handling and availability checks

Validate and normalize custom TTS server URLs before proxying requests.
Gate server TTS availability by provider mode to avoid OpenAI/custom misrouting.
Keep browser and message TTS hooks aligned with the new provider-specific checks.

packages/ui/src/hooks/useMessageTTS.ts

@@ -44,6 +44,7 @@ export function useMessageTTS(): UseMessageTTSReturn {
 
     const { speak: speakServerTTS, stop: stopServerTTS, isAvailable: isServerTTSAvailable } = useServerTTS({
         enabled: shouldCheckOpenAIAvailability,
+        availabilityMode: voiceProvider === 'openai-compatible' ? 'openai-compatible' : 'openai',
     });
     const { speak: speakSayTTS, stop: stopSayTTS, isAvailable: isSayTTSAvailable } = useSayTTS({
         enabled: shouldCheckSayAvailability,