fix: serve /webfonts static dir - icons missing after FA localization

CSS at /css/fontawesome.min.css references url(../webfonts/...) which
resolves to /webfonts/*.woff2; server.js only mounted /css and /js,
so all font requests 404'd and every icon silently disappeared.

backend/server.js

@@ -115,6 +115,8 @@ const staticOpts = {
 };
 app.use('/css', express.static(path.join(PUBLIC_DIR, 'css'), staticOpts));
 app.use('/js', express.static(path.join(PUBLIC_DIR, 'js'), staticOpts));
+// Font Awesome 本地化: 字体文件(缺失会导致图标全部无法显示)
+app.use('/webfonts', express.static(path.join(PUBLIC_DIR, 'webfonts'), staticOpts));
 
 const installRoutes = require('./routes/install');
 app.use('/api/install', installRoutes);