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.

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

web_src/css/index.css

@@ -20,6 +20,7 @@
 @import "./modules/modal.css";
 @import "./modules/tab.css";
 @import "./modules/form.css";
+@import "./modules/dropdown.css";
 @import "./modules/shortcut.css";
 
 @import "./modules/tippy.css";