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 = {