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.test.ts

@@ -5,10 +5,23 @@ import {
   browserNavigateRequestSchema,
   browserStopLoadingRequestSchema,
   builtinEmbeddingConnectionId,
+  clipboardTextSchema,
   legacyEmbeddingConnectionId,
   runtimeSettingsInputSchema
 } from './contracts'
 
+describe('clipboard text contract', () => {
+  it('accepts text and rejects non-text input', () => {
+    expect(clipboardTextSchema.parse('Markdown response')).toBe(
+      'Markdown response'
+    )
+    expect(clipboardTextSchema.parse('')).toBe('')
+    expect(clipboardTextSchema.safeParse({ text: 'no' }).success).toBe(
+      false
+    )
+  })
+})
+
 describe('browser control contracts', () => {
   it('requires authoritative toolbar metadata and accepts committed URLs up to 8192 characters', () => {
     const base = {

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

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',