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- Rename Calendar tab to Synchronization with two sections: * Calendar Sync (Google, Apple, CalDAV, ICS) * Contact Sync (CardDAV) - NEW - Add visual tab grouping with CSS separators between sections - Implement CardDAV account management UI: * Add/delete accounts * Enable/disable addressbooks * Manual sync trigger * Connection testing - Add UX improvements: * Status badges (success/error/syncing) * Empty states with onboarding * Inline help tooltips (prepared) * Breadcrumb navigation (prepared) - Update i18n keys in all 14 locales - All 109 tests passing Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>