captcha: fix for custom invite form subpath

ts/modules/captcha.ts

@@ -1,5 +1,7 @@
 import { _get, _post } from "./common.js";
 
+declare var window: GlobalWindow;
+
 export class Captcha {
     isPWR = false;
     enabled = true;
@@ -51,7 +53,7 @@ export class Captcha {
                 this.captchaID = this.isPWR ? this.code : req.response["id"];
                 // the Math.random() appearance below is used for PWRs, since they don't have a unique captchaID. The parameter is ignored by the server, but tells the browser to reload the image.
                 document.getElementById("captcha-img").innerHTML = `
-                <img class="w-full" src="${window.location.toString().substring(0, window.location.toString().lastIndexOf("/invite"))}/captcha/img/${this.code}/${this.isPWR ? Math.random() : this.captchaID}${this.isPWR ? "?pwr=true" : ""}"></img>
+                <img class="w-full" src="${window.location.toString().substring(0, window.location.toString().lastIndexOf(window.pages.Form))}/captcha/img/${this.code}/${this.isPWR ? Math.random() : this.captchaID}${this.isPWR ? "?pwr=true" : ""}"></img>
                 `;
                 this.input.value = "";
             }