Add a unified welcome modal for guest mode that displays:
- App icon, name, and description from shared-branding
- Feature list of what guests can do (localized DE/EN)
- Warning about local-only data storage
- Login, Register, Help, and "Continue as Guest" buttons
New files:
- GuestWelcomeModal.svelte - The modal component
- guestWelcome.ts - localStorage utilities for tracking seen state
Integrated into: contacts, chat, todo, calendar, and clock apps
