refactor(webui): shrink fork implementation

webui/src/i18n/locales/zh-TW/common.json

@@ -811,10 +811,7 @@
     },
     "scrollToBottom": "捲動到底部",
     "loadEarlier": "載入更早訊息",
-    "fork": {
-      "failed": "無法分叉這個對話，請重試。",
-      "fromHistory": "從歷史訊息分叉"
-    },
+    "forkedFromHistory": "從歷史訊息分叉",
     "promptNavigator": {
       "open": "開啟輸入導覽",
       "title": "輸入列表",
@@ -840,8 +837,6 @@
     "agentActivityLiveSummary": "進行中… · {{reasoning}} 步 · {{tools}} 次工具呼叫",
     "agentActivityLiveToolsOnly": "進行中… · {{tools}} 次工具呼叫",
     "imageAttachment": "圖片附件",
-    "copyMessage": "複製",
-    "copiedMessage": "已複製",
     "forkFromHere": "分叉",
     "copyReply": "複製",
     "copiedReply": "已複製",