Move Fomantic dropdown CSS to custom module

Moved fomantic dropdown css to custom module, tested on the dropdown
devtest page, it renders exactly the same as before while using roughly
half the 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.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

web_src/fomantic/build/fomantic.css

@@ -1,3 +1,2 @@
-@import "./components/dropdown.css";
 @import "./components/modal.css";
 @import "./components/search.css";

web_src/fomantic/semantic.json

@@ -22,7 +22,6 @@
   "admin": false,
   "components": [
     "api",
-    "dropdown",
     "modal",
     "search",
     "tab"