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>

stylelint.config.js

@@ -125,7 +125,7 @@ export default {
     'csstools/value-no-unknown-custom-properties': [true, {importFrom: cssVarFiles}],
     'declaration-block-no-duplicate-properties': [true, {ignore: ['consecutive-duplicates-with-different-values']}],
     'declaration-block-no-redundant-longhand-properties': [true, {ignoreShorthands: ['flex-flow', 'overflow', 'grid-template']}],
-    'declaration-property-unit-disallowed-list': {'line-height': ['em']},
+    'declaration-property-unit-disallowed-list': null,
     'declaration-property-value-disallowed-list': {'word-break': ['break-word']},
     'font-family-name-quotes': 'always-where-recommended',
     'function-name-case': 'lower',