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>

templates/user/auth/captcha.tmpl

@@ -19,7 +19,10 @@
 {{else if eq .CaptchaType "mcaptcha"}}
 	<div class="inline field tw-text-center">
 		<div class="m-captcha-style" id="mcaptcha__widget-container"></div>
-		<div id="captcha" data-captcha-type="m-captcha" data-sitekey="{{.McaptchaSitekey}}" data-instance-url="{{.McaptchaURL}}"></div>
+		<label id="mcaptcha__token-label" hidden data-mcaptcha_url="{{.McaptchaURL}}/widget?sitekey={{.McaptchaSitekey}}">
+			<input id="mcaptcha__token" name="mcaptcha__token">{{/* the id and name are hard-coded in the library, cant't be changed */}}
+		</label>
+		<div id="captcha" data-captcha-type="m-captcha"></div>
 	</div>
 {{else if eq .CaptchaType "cfturnstile"}}
 	<div class="inline field tw-text-center">