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

renderer/renderer.js

@@ -128,6 +128,29 @@
     }
   }
 
+  // Intercept link clicks in preview — open in system browser
+  function initPreviewLinks() {
+    preview.addEventListener('click', (e) => {
+      const link = e.target.closest('a');
+      if (!link) return;
+      e.preventDefault();
+      const href = link.getAttribute('href');
+      if (!href) return;
+      // Anchor links: scroll within preview
+      if (href.startsWith('#')) {
+        const target = preview.querySelector(href);
+        if (target) target.scrollIntoView({ behavior: 'smooth' });
+        return;
+      }
+      // External links: open in system browser
+      if (typeof window.electronAPI !== 'undefined' && window.electronAPI.openExternal) {
+        window.electronAPI.openExternal(href);
+      } else {
+        window.open(href, '_blank', 'noopener');
+      }
+    });
+  }
+
   // ===== Line Numbers =====
   function updateLineNumbers() {
     const count = editor.value.split('\n').length;
@@ -663,7 +686,11 @@
         return;
       }
       const shouldClose = confirm('有文件尚未保存，确定要关闭吗？');
-      if (shouldClose) window.electronAPI.forceClose();
+      if (shouldClose) {
+        window.electronAPI.forceClose();
+      } else {
+        window.electronAPI.cancelClose();
+      }
     });
   }
 
@@ -695,6 +722,13 @@
     btnWelcomeNew.addEventListener('click', () => createNewTab(null, ''));
 
     if (typeof window.electronAPI !== 'undefined') {
+      // Remove old listeners first to prevent stacking
+      window.electronAPI.removeAllListeners('file:openInTab');
+      window.electronAPI.removeAllListeners('file:opened');
+      window.electronAPI.removeAllListeners('menu:save');
+      window.electronAPI.removeAllListeners('menu:saveAs');
+      window.electronAPI.removeAllListeners('menu:viewMode');
+
       // Main process sends file to open in a new tab
       window.electronAPI.onFileOpenInTab((data) => {
         createNewTab(data.filePath, data.content);
@@ -719,6 +753,7 @@
     initDragDrop();
     initKeyboard();
     initEditorTab();
+    initPreviewLinks();
     initEventListeners();
     initFileWatch();
     initUnsavedWarning();