Rename `buttons` module -> `frontend-shared`

This makes the structure consistent with LMS' styles

src/styles/sidebar/buttons.scss → src/styles/sidebar/frontend-shared.scss

-@use 'sass:map';
+// Extensions and/or overrides to shared component styles from the
+// `@hypothesis/frontend-shared` package
 
 @use '../variables' as var;
 

src/styles/sidebar/sidebar.scss

@@ -10,8 +10,8 @@
 // -----------------
 @use '@hypothesis/frontend-shared/styles';
 
-// Custom button styling for the application
-@use './buttons';
+// Extensions and overrides to patterns from `frontend-shared`
+@use './frontend-shared';
 
 // Local patterns
 @use '../patterns';

src/styles/ui-playground/ui-playground.scss

 // Shared pattern-library styles
 @use '@hypothesis/frontend-shared/styles/pattern-library';
 
-// This app pattern styles
-@use '../sidebar/buttons';
+// Local overrides to frontend-shared styling
+@use '../sidebar/frontend-shared';