Web polish from testing feedback + admin role toggle

- style the settings-table dropdowns for the dark theme (were white)
- share-icon picker on one even row (no lopsided wrap)
- more breathing room between colour hex and swatch
- user management: promote/demote admin (new PUT /users/{id}/admin,
  guarded against self-change and removing the last admin)
- calendar management table: clip/ellipsis cells so narrow columns no
  longer overlap (full-width detail rows still wrap)
- centralise default colours: utils.applyTheme now derives every fallback
  from settings-sync.DEFAULT_COLORS (single place for coders to edit)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -212,6 +212,7 @@ const translations = {
 
     // User management
     users_add: 'Benutzer hinzufügen', users_is_admin: 'Administrator',
+    users_make_admin: 'Zu Admin machen', users_revoke_admin: 'Admin entziehen',
 
     // Profile
     profile_title: 'Profil', profile_upload: 'Bild hochladen',
@@ -564,6 +565,7 @@ const translations = {
 
     // User management
     users_add: 'Add user', users_is_admin: 'Administrator',
+    users_make_admin: 'Make admin', users_revoke_admin: 'Revoke admin',
 
     // Profile
     profile_title: 'Profile', profile_upload: 'Upload image',