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/preload/index.ts

@@ -254,6 +254,11 @@ const desktopApi: DesktopApi = {
       return () => ipcRenderer.removeListener(ipcChannels.settingsOpen, handler)
     }
   },
+  clipboard: {
+    writeText: async (text: string) => {
+      await ipcRenderer.invoke(ipcChannels.clipboardWriteText, text)
+    }
+  },
   agent: {
     getStatus: (selection) =>
       ipcRenderer.invoke(

src/preload/preload-sandbox.test.ts

@@ -21,6 +21,20 @@ describe('sandboxed preload', () => {
     expect(source).not.toMatch(/\bfrom\s+['"]zod['"]/u)
   })
 
+  it('exposes only explicit clipboard text writing', () => {
+    const source = readFileSync(
+      join(process.cwd(), 'src', 'preload', 'index.ts'),
+      'utf8'
+    )
+    const clipboard =
+      source.match(
+        /clipboard: \{(?<body>[\s\S]*?)\r?\n {2}\},\r?\n {2}agent:/u
+      )?.groups?.body ?? ''
+    expect(clipboard).toContain('writeText: async (text: string)')
+    expect(clipboard).toContain('ipcChannels.clipboardWriteText')
+    expect(clipboard).not.toMatch(/\b(?:read|image|html|rtf|bookmark)\b/iu)
+  })
+
   it('exposes only narrow browser-control methods', () => {
     const source = readFileSync(
       join(process.cwd(), 'src', 'preload', 'index.ts'),