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>

services/context/captcha.go

@@ -48,7 +48,7 @@ func GetImageCaptcha() *captcha.Captcha {
 const (
 	gRecaptchaResponseField  = "g-recaptcha-response"
 	hCaptchaResponseField    = "h-captcha-response"
-	mCaptchaResponseField    = "m-captcha-response"
+	mCaptchaResponseField    = "mcaptcha__token" // this form key is hard-coded in the mcaptcha frontend library
 	cfTurnstileResponseField = "cf-turnstile-response"
 )