fix: 重置默认场景为 Scene A，更新初始演示数据

src/context/ChatContext.tsx

@@ -13,8 +13,8 @@ interface ChatContextValue {
 const ChatContext = createContext<ChatContextValue | null>(null)
 
 export function ChatProvider({ children }: { children: ReactNode }) {
-  const [activeDemo, setActiveDemo] = useState(4) // Default: Scene F
-  const [envelope, setEnvelope] = useState<PromptEnvelope>(demos[4].envelope)
+  const [activeDemo, setActiveDemo] = useState(0) // Default: Scene A
+  const [envelope, setEnvelope] = useState<PromptEnvelope>(demos[0].envelope)
 
   const switchDemo = (i: number) => {
     setActiveDemo(i)