Enhance voice interaction features and introduce voice preview functionality

- Update README to reflect the integration of the DebugVoicePanel with WebSocket support for voice interactions.
- Refactor voice_webrtc.py to improve error handling during WebRTC signaling and include assistant_id in the offer payload.
- Add useVoicePreview hook to manage microphone access and WebRTC connections for real-time voice previews.
- Modify AssistantPage to incorporate new visualizer options and pass assistantId to DebugVoicePanel, enhancing user experience during audio interactions.
- Update API model to include new fields for voice, speed, and language, supporting TTS and ASR configurations.

frontend/src/lib/api.ts

@@ -5,7 +5,7 @@
  * 注意:api_key 读取时后端永远打码,写回打码占位符表示"不改 key"(写时哨兵)。
  */
 
-const API_BASE =
+export const API_BASE =
   process.env.NEXT_PUBLIC_API_BASE_URL ?? "http://localhost:8000";
 
 export type ModelType = "LLM" | "ASR" | "TTS" | "Realtime" | "Embedding";
@@ -34,6 +34,9 @@ export type CredentialUpsert = {
   interfaceType: InterfaceType;
   apiUrl: string;
   apiKey: string;
+  voice: string;
+  speed: number;
+  language: string;
   isDefault: boolean;
 };