✨ feat(shared-auth-ui): add GuestWelcomeModal for guest onboarding

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

packages/shared-auth-ui/src/index.ts

@@ -6,6 +6,7 @@ export { default as ForgotPasswordPage } from './pages/ForgotPasswordPage.svelte
 // Components
 export { default as GoogleSignInButton } from './components/GoogleSignInButton.svelte';
 export { default as AppleSignInButton } from './components/AppleSignInButton.svelte';
+export { default as GuestWelcomeModal } from './components/GuestWelcomeModal.svelte';
 
 // Utilities
 export {
@@ -28,10 +29,17 @@ export {
 	type AppleAuthorizationResponse,
 } from './utils/appleAuth';
 
-// Types
-export type { AuthUIConfig, AuthServiceInterface, AuthResult } from './types';
+export {
+	shouldShowGuestWelcome,
+	markGuestWelcomeSeen,
+	resetGuestWelcome,
+	resetAllGuestWelcome,
+} from './utils/guestWelcome';
 
-// Page Translation Types
-export type { LoginTranslations } from './pages/LoginPage.svelte';
-export type { RegisterTranslations } from './pages/RegisterPage.svelte';
-export type { ForgotPasswordTranslations } from './pages/ForgotPasswordPage.svelte';
+// Types
+export type {
+	AuthUIConfig,
+	AuthServiceInterface,
+	AuthResult,
+	GuestWelcomeTranslations,
+} from './types';