refactor: move go-chi/captcha into Gitea (#39529)

The `gitea.com/go-chi/captcha` package only exists for Gitea,
so it moves into `modules/imagecaptcha`.

- Reloading an expired challenge shows a new image instead of a broken one
- Every answer is consumed on its first check
- OpenID registration stops after a failed captcha

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/captcha.ts

@@ -1,6 +1,13 @@
 import {isDarkTheme} from '../utils.ts';
+import {registerGlobalEventFunc} from '../modules/observer.ts';
 
 export async function initCaptcha() {
+  registerGlobalEventFunc('click', 'onImageCaptchaReload', (img: HTMLImageElement) => {
+    const url = new URL(img.src);
+    url.searchParams.set('reload', String(Date.now()));
+    img.src = url.href;
+  });
+
   const captchaEl = document.querySelector('#captcha');
   if (!captchaEl) return;