fix: 打開不同對話時, 使用了同一個 ChatFragment

* 並修復了使用 key 時, 因爲卸載組件后 ref 丟失導致的錯誤

client/ui/App.tsx

@@ -173,7 +173,8 @@ export default function App() {
                 }
                 {
                     isShowChatFragment && <ChatFragment
-                        target={currentChatId} />
+                        target={currentChatId}
+                        key={currentChatId} />
                 }
             </div>
         </div>

client/ui/AppMobile.tsx

@@ -109,6 +109,7 @@ export default function AppMobile() {
                     <ChatFragment
                         showReturnButton={true}
                         onReturnButtonClicked={() => setIsShowChatFragment(false)}
+                        key={currentChatId}
                         target={currentChatId} />
                 </div>
             </mdui-dialog>

client/ui/useEventListener.ts

@@ -3,6 +3,6 @@ import * as React from 'react'
 export default function useEventListener<T extends HTMLElement | null>(ref: React.MutableRefObject<T>, eventName: string, callback: (event: Event) => void) {
     React.useEffect(() => {
         ref.current!.addEventListener(eventName, callback)
-        return () => ref.current!.removeEventListener(eventName, callback)
+        return () => ref.current?.removeEventListener(eventName, callback)
     }, [ref, eventName, callback])
 }