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/shared/ipc-channels.ts

@@ -11,6 +11,7 @@ export const ipcChannels = {
   appRendererPersistenceRequest: 'app:renderer-persistence-request',
   appRendererPersistenceComplete: 'app:renderer-persistence-complete',
   appClearLocalData: 'app:clear-local-data',
+  clipboardWriteText: 'clipboard:write-text',
   conversationNew: 'conversation:new',
   settingsOpen: 'settings:open',
   agentStatus: 'agent:get-status',