feat: 限制只能打开 md/txt 文件，新建文件仅保存为 md

- 打开对话框移除「所有文件」选项
- 保存对话框移除 .txt 选项
- 拖拽打开增加扩展名过滤
- 浏览器降级路径同步过滤

renderer/renderer.js

@@ -429,6 +429,8 @@
       input.multiple = true;
       input.onchange = (e) => {
         Array.from(e.target.files).forEach(file => {
+          const ext = file.name.substring(file.name.lastIndexOf('.')).toLowerCase();
+          if (!['.md', '.markdown', '.txt'].includes(ext)) return;
           const reader = new FileReader();
           reader.onload = (ev) => {
             createNewTab(file.name, ev.target.result);
@@ -584,8 +586,11 @@
       dropOverlay.classList.add('hidden');
 
       const files = e.dataTransfer.files;
+      const allowedExts = ['.md', '.markdown', '.txt'];
       for (const file of files) {
         const filePath = file.path;
+        const ext = filePath ? filePath.substring(filePath.lastIndexOf('.')).toLowerCase() : '';
+        if (!allowedExts.includes(ext)) continue;
         if (filePath && typeof window.electronAPI !== 'undefined') {
           const result = await window.electronAPI.readFile(filePath);
           if (result.success) {