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/sw.js

@@ -7,7 +7,7 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v26';
+const CACHE_VERSION  = 'calendarr-v27';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {