fix(panel): register /groups SPA route so hard refresh returns index.html

The frontend has a groups page route and sidebar entry, but the backend
never registered a GET handler for /panel/groups. A hard browser refresh
on that page fell through to the 404 handler. Add the missing panelSPA
registration alongside the other page routes.

Fixes #4837

web/controller/xui.go

@@ -39,6 +39,7 @@ func (a *XUIController) initRouter(g *gin.RouterGroup) {
 	g.GET("/", a.panelSPA)
 	g.GET("/inbounds", a.panelSPA)
 	g.GET("/clients", a.panelSPA)
+	g.GET("/groups", a.panelSPA)
 	g.GET("/nodes", a.panelSPA)
 	g.GET("/settings", a.panelSPA)
 	g.GET("/xray", a.panelSPA)