fix: 预览选择复制、关闭超时、链接跳转、监听器泄漏

1. 预览区无法选择复制 — #preview 添加 user-select: text
2. IPC 监听器重复注册 — 注册前先 removeAllListeners 防止堆叠
3. 关闭确认取消后 5s 超时仍强制关闭 — 新增 cancelClose IPC 清除超时
4. 预览区链接点击离开应用 — 拦截点击，通过 shell.openExternal 打开
5. 预览区链接光标样式 — .markdown-body a 添加 cursor: pointer

preload.js

@@ -1,4 +1,4 @@
-const { contextBridge, ipcRenderer } = require('electron');
+const { contextBridge, ipcRenderer, shell } = require('electron');
 
 contextBridge.exposeInMainWorld('electronAPI', {
   // File operations
@@ -15,6 +15,10 @@ contextBridge.exposeInMainWorld('electronAPI', {
 
   // Window control
   forceClose: () => ipcRenderer.invoke('window:forceClose'),
+  cancelClose: () => ipcRenderer.invoke('window:cancelClose'),
+
+  // Shell
+  openExternal: (url) => shell.openExternal(url),
 
   // Events from main process
   onFileOpenInTab: (callback) => ipcRenderer.on('file:openInTab', (event, data) => callback(data)),