Web: unified settings sync table with per-setting sync toggles

Replace the ad-hoc appearance panel with a uniform table (sync icon | name |
value): dropdowns instead of button groups, every colour with a reset to a
canonical default, a per-row sync toggle, and a global "sync all" master
switch.

- new settings-sync.js: canonical default colours, declarative table
  definition, browser-local value copy, effective-value resolution
- calendar.js: generalised sync engine (pull applies only synced keys from the
  server, save pushes only synced keys + the flag map), unified the two save
  buttons into one, dropped the dead text/line-contrast code, moved default
  duration into the appearance table, effective UI language on load
- account settings (private visibility, group calendar, hide profile) stay in a
  dedicated profile block, not the sync table
- app.css: settings table / sync toggle / sync icon styles
- i18n: sync-all / sync-this / share-icon strings; sw cache bump

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-v25';
+const CACHE_VERSION  = 'calendarr-v26';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {