Fix Citation modal responsiveness and clipboard copy (#29799)

The modal was broken in two ways:

- On small screens, the input box was partially hanging outside the
modal. Fixed with flexbox and increased modal width.
- The clipboard copy was not working because the modal had both
`data-clipboard-text` and `data-clipboard-target`, while we only support
one of those. Made a small tweak in clipboard as well so that it will
still fall back to target if text is empty.

(cherry picked from commit 94512ee0628dc0d2b697441a4355ace54b6515cd)

web_src/js/features/clipboard.js

@@ -15,10 +15,8 @@ export function initGlobalCopyToClipboardListener() {
 
     e.preventDefault();
 
-    let text;
-    if (target.hasAttribute('data-clipboard-text')) {
-      text = target.getAttribute('data-clipboard-text');
-    } else {
+    let text = target.getAttribute('data-clipboard-text');
+    if (!text) {
       text = document.querySelector(target.getAttribute('data-clipboard-target'))?.value;
     }