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/js/i18n.js

@@ -65,6 +65,10 @@ const translations = {
     settings_nav_calendars: 'Kalender',
     settings_nav_google: 'Google Konten',
     settings_nav_users: 'Benutzerverwaltung',
+    settings_sync_all: 'Alle synchronisieren',
+    settings_sync_all_desc: 'Diese Einstellungen zwischen deinen Geräten teilen',
+    settings_sync_this: 'Zwischen Geräten synchronisieren',
+    settings_share_icon: 'Teilen-Symbol',
     settings_colors: 'Farben',
     settings_primary_color: 'Primärfarbe', settings_accent_color: 'Akzentfarbe',
     settings_today_color: 'Heutige-Tag-Farbe',
@@ -413,6 +417,10 @@ const translations = {
     settings_nav_calendars: 'Calendars',
     settings_nav_google: 'Google Accounts',
     settings_nav_users: 'User Management',
+    settings_sync_all: 'Sync all',
+    settings_sync_all_desc: 'Share these settings across your devices',
+    settings_sync_this: 'Sync across devices',
+    settings_share_icon: 'Share icon',
     settings_colors: 'Colors',
     settings_primary_color: 'Primary color', settings_accent_color: 'Accent color',
     settings_today_color: 'Today highlight color',