feat: add copy action to AI responses

Completed AI responses now expose a bottom copy action that copies their Markdown source without reasoning, tool logs, or citation metadata. Clipboard writes use a validated Preload/Main bridge so sandboxed Electron sessions no longer fail with a renderer clipboard permission error; full-conversation copy uses the same path.

Release note: AI 回复底部现在可以直接复制 Markdown 原文，并修复了沙箱环境下回复和完整会话可能无法写入剪贴板的问题。

src/preload/index.ts

@@ -254,6 +254,11 @@ const desktopApi: DesktopApi = {
       return () => ipcRenderer.removeListener(ipcChannels.settingsOpen, handler)
     }
   },
+  clipboard: {
+    writeText: async (text: string) => {
+      await ipcRenderer.invoke(ipcChannels.clipboardWriteText, text)
+    }
+  },
   agent: {
     getStatus: (selection) =>
       ipcRenderer.invoke(