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/contracts.ts

@@ -1116,6 +1116,8 @@ export type ContextFileSelectionProgress = {
 
 export const maximumPastedImageBytes = 12 * 1024 * 1024
 
+export const clipboardTextSchema = z.string()
+
 export const pastedImageInputSchema = z
   .object({
     data: z
@@ -1561,6 +1563,9 @@ export type DesktopApi = {
     onNewConversation: (listener: () => void) => () => void
     onOpenSettings: (listener: () => void) => () => void
   }
+  clipboard: {
+    writeText: (text: string) => Promise<void>
+  }
   agent: {
     getStatus: (
       selection?: AgentRuntimeSelection