Move Fomantic dropdown CSS to custom module (#36530)

Moved fomantic dropdown css to custom module, tested on the dropdown
devtest page, it renders exactly the same as before while using roughly
50% less CSS. The clean up was very conservative, likely more can be
done in the future.

Also, this fixes a bug present on main branch where dropdown border has
incorrect color on hover.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

web_src/fomantic/build/components/dropdown.js

@@ -4158,7 +4158,7 @@ $.fn.dropdown.settings.templates = {
       html        = '',
       escape = $.fn.dropdown.settings.templates.escape
     ;
-    html +=  '<i class="dropdown icon"></i>';
+    html +=  '<i class="dropdown icon"><svg viewBox="0 0 16 16" class="svg octicon-triangle-down" aria-hidden="true" width="14" height="14"><path d="m4.427 7.427 3.396 3.396a.25.25 0 0 0 .354 0l3.396-3.396A.25.25 0 0 0 11.396 7H4.604a.25.25 0 0 0-.177.427"></path></svg></i>';
     if(placeholder) {
       html += '<div class="default text">' + escape(placeholder,preserveHTML) + '</div>';
     }