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>

web_src/js/features/captcha.ts

@@ -34,22 +34,10 @@ export async function initCaptcha() {
       break;
     }
     case 'm-captcha': {
-      const mCaptcha = await import('@mcaptcha/vanilla-glue');
-
-      // FIXME: the mCaptcha code is not right, it's a miracle that the wrong code could run
-      // * the "vanilla-glue" has some problems with es6 module.
-      // * the INPUT_NAME is a "const", it should not be changed.
-      // * the "mCaptcha.default" is actually the "Widget".
-
-      mCaptcha.INPUT_NAME = 'm-captcha-response';
-      const instanceURL = captchaEl.getAttribute('data-instance-url')!;
-
-      new mCaptcha.default({
-        siteKey: {
-          instanceUrl: new URL(instanceURL),
-          key: siteKey,
-        },
-      });
+      // ref: https://github.com/mCaptcha/glue/blob/master/packages/vanilla/README.md
+      // sample: https://github.com/mCaptcha/glue/blob/master/packages/vanilla/static/embeded.html
+      // @mcaptcha/vanilla-glue 0.1.0-rc2 auto-runs on module load, use the existing elements to render.
+      await import('@mcaptcha/vanilla-glue');
       break;
     }
     default: