Fix mCaptcha broken after Vite migration (#37492)

After the Webpack-to-Vite migration (#37002), mCaptcha stopped working
entirely on the registration page, throwing an error:

`TypeError: setting getter-only property "INPUT_NAME"`

This fix stops trying to mutate the read-only INPUT_NAME export. Instead
it probes for the Widget constructor at module.default (direct) or
module.default.default (CJS-wrapped), constructs the widget, and then
renames the hidden input element it creates to m-captcha-response which
is the field name Gitea's backend reads from the submitted form.

Generative AI was used to help with making this PR.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/web/auth/auth.go

@@ -64,7 +64,7 @@ func prepareCommonAuthPageData(ctx *context.Context, opt CommonAuthOptions) {
 		ctx.Data["RecaptchaSitekey"] = setting.Service.RecaptchaSitekey
 		ctx.Data["HcaptchaSitekey"] = setting.Service.HcaptchaSitekey
 		ctx.Data["McaptchaSitekey"] = setting.Service.McaptchaSitekey
-		ctx.Data["McaptchaURL"] = setting.Service.McaptchaURL
+		ctx.Data["McaptchaURL"] = strings.TrimSuffix(setting.Service.McaptchaURL, "/")
 		ctx.Data["CfTurnstileSitekey"] = setting.Service.CfTurnstileSitekey
 		if setting.Service.CaptchaType == setting.ImageCaptcha {
 			ctx.Data["Captcha"] = context.GetImageCaptcha()