Move `form-input` styles to `components/form-input.scss`

Follow the module naming convention of other components.

src/styles/forms.scss → src/styles/sidebar/components/form-input.scss

-@use "mixins/forms";
-@use "variables" as var;
+@use "../../mixins/forms";
+@use "../../variables" as var;
 
 .form-input {
   @include forms.form-input;

src/styles/sidebar/sidebar.scss

@@ -39,6 +39,7 @@
 @use './components/button';
 @use './components/excerpt';
 @use './components/focused-mode-header';
+@use './components/form-input';
 @use './components/group-list';
 @use './components/group-list-item';
 @use './components/help-panel';