🐛 加强文件名解码的空值处理

- 后端decodeHtmlEntities添加空字符串默认值
- 前端decodeHtmlEntities非字符串时返回空字符串
- getFileDisplayName增强类型检查

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/app.js

@@ -2017,7 +2017,7 @@ handleDragLeave(e) {
 
     // HTML实体解码（前端兜底，防止已实体化的文件名显示乱码）
     decodeHtmlEntities(str) {
-      if (typeof str !== 'string') return str;
+      if (typeof str !== 'string') return '';
       const entityMap = {
         amp: '&',
         lt: '<',
@@ -2052,9 +2052,11 @@ handleDragLeave(e) {
 
     getFileDisplayName(file) {
       if (!file) return '';
-      const base = file.displayName || file.name || '';
+      const base = (typeof file.displayName === 'string' && file.displayName !== '')
+        ? file.displayName
+        : (typeof file.name === 'string' ? file.name : '');
       const decoded = this.decodeHtmlEntities(base);
-      return decoded || base;
+      return decoded || base || '';
     },
 
     openShare(url) {