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

main.js

@@ -90,8 +90,7 @@ async function showOpenDialog() {
   const result = await dialog.showOpenDialog(mainWindow, {
     properties: ['openFile'],
     filters: [
-      { name: 'Markdown 文件', extensions: ['md', 'markdown', 'txt'] },
-      { name: '所有文件', extensions: ['*'] }
+      { name: 'Markdown 文件', extensions: ['md', 'markdown', 'txt'] }
     ]
   });
   if (!result.canceled && result.filePaths.length > 0) {
@@ -169,8 +168,7 @@ ipcMain.handle('file:save', async (event, { filePath, content }) => {
     } else {
       const result = await dialog.showSaveDialog(mainWindow, {
         filters: [
-          { name: 'Markdown 文件', extensions: ['md'] },
-          { name: '文本文件', extensions: ['txt'] }
+          { name: 'Markdown 文件', extensions: ['md'] }
         ]
       });
       if (!result.canceled) {
@@ -192,8 +190,7 @@ ipcMain.handle('file:saveAs', async (event, { content }) => {
   try {
     const result = await dialog.showSaveDialog(mainWindow, {
       filters: [
-        { name: 'Markdown 文件', extensions: ['md'] },
-        { name: '文本文件', extensions: ['txt'] }
+        { name: 'Markdown 文件', extensions: ['md'] }
       ]
     });
     if (!result.canceled) {