Remove jQuery from username change prompt and fix its detection (#29197)

- Switched to plain JavaScript
- Tested the user rename prompt toggling functionality and it works as
before
- Fixed bug that allowed pasting with the mouse to avoid the prompt

# Before

![before](https://github.com/go-gitea/gitea/assets/20454870/aa300ad7-612b-461e-bbb2-3f74b3b83ede)

# After

![after](https://github.com/go-gitea/gitea/assets/20454870/f2b5a51b-7b39-43c7-8a4a-62f1f77acae4)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/js/features/user-settings.js

@@ -1,18 +1,19 @@
-import $ from 'jquery';
 import {hideElem, showElem} from '../utils/dom.js';
 
 export function initUserSettings() {
-  if ($('.user.settings.profile').length > 0) {
-    $('#username').on('keyup', function () {
-      const $prompt = $('#name-change-prompt');
-      const $prompt_redirect = $('#name-change-redirect-prompt');
-      if ($(this).val().toString().toLowerCase() !== $(this).data('name').toString().toLowerCase()) {
-        showElem($prompt);
-        showElem($prompt_redirect);
-      } else {
-        hideElem($prompt);
-        hideElem($prompt_redirect);
-      }
-    });
-  }
+  if (document.querySelectorAll('.user.settings.profile').length === 0) return;
+
+  const usernameInput = document.getElementById('username');
+  if (!usernameInput) return;
+  usernameInput.addEventListener('input', function () {
+    const prompt = document.getElementById('name-change-prompt');
+    const promptRedirect = document.getElementById('name-change-redirect-prompt');
+    if (this.value.toLowerCase() !== this.getAttribute('data-name').toLowerCase()) {
+      showElem(prompt);
+      showElem(promptRedirect);
+    } else {
+      hideElem(prompt);
+      hideElem(promptRedirect);
+    }
+  });
 }