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/main/ipc.ts

@@ -1,6 +1,7 @@
 import {
   app,
   BrowserWindow,
+  clipboard,
   dialog,
   ipcMain,
   shell
@@ -31,6 +32,7 @@ import {
   browserReloadRequestSchema,
   browserStopLoadingRequestSchema,
   browserStopRequestSchema,
+  clipboardTextSchema,
   conversationQueueUserInputSchema,
   defaultRuntimeSettings,
   knowledgeCreateSchema,
@@ -3298,6 +3300,11 @@ export function registerIpcHandlers(
     }
   })
 
+  registerHandler(ipcChannels.clipboardWriteText, (event, input) => {
+    assertTrustedSender(event, window)
+    clipboard.writeText(clipboardTextSchema.parse(input))
+  })
+
   registerHandler(
     ipcChannels.appRendererPersistenceReady,
     (event) => {