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

main.js

@@ -6,6 +6,7 @@ let mainWindow = null;
 let activeFilePath = null; // Currently active tab's file path
 let pendingFilePath = null;
 let fileWatcher = null;
+let closeTimeout = null;
 
 function createWindow() {
   mainWindow = new BrowserWindow({
@@ -48,7 +49,8 @@ function createWindow() {
       mainWindow.close();
       return;
     }
-    setTimeout(() => {
+    // 5s safety timeout in case renderer is unresponsive
+    closeTimeout = setTimeout(() => {
       if (mainWindow && !mainWindow.isDestroyed()) {
         mainWindow.removeAllListeners('close');
         mainWindow.close();
@@ -237,11 +239,22 @@ ipcMain.handle('tab:switched', (event, filePath) => {
 });
 
 ipcMain.handle('window:forceClose', () => {
+  if (closeTimeout) {
+    clearTimeout(closeTimeout);
+    closeTimeout = null;
+  }
   stopWatching();
   mainWindow.removeAllListeners('close');
   mainWindow.close();
 });
 
+ipcMain.handle('window:cancelClose', () => {
+  if (closeTimeout) {
+    clearTimeout(closeTimeout);
+    closeTimeout = null;
+  }
+});
+
 // Command line file
 function getCommandLineFile() {
   const args = process.argv.slice(1);