✨ 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

apps/chat/apps/web/src/routes/(protected)/+layout.svelte

@@ -25,6 +25,7 @@
 	import { getLanguageDropdownItems, getCurrentLanguageLabel } from '@manacore/shared-i18n';
 	import { setLocale, supportedLocales } from '$lib/i18n';
 	import AuthGateModal from '$lib/components/AuthGateModal.svelte';
+	import { GuestWelcomeModal, shouldShowGuestWelcome } from '@manacore/shared-auth-ui';
 	import type { LayoutData } from './$types';
 
 	// App switcher items
@@ -40,6 +41,9 @@
 	let showAuthGateModal = $state(false);
 	let authGateAction = $state<'save' | 'sync' | 'ai' | 'feature'>('ai');
 
+	// Guest welcome modal state
+	let showGuestWelcome = $state(false);
+
 	// Check if in guest mode
 	let isGuestMode = $derived(!authStore.isAuthenticated);
 	let sessionConversationCount = $derived(sessionConversationsStore.count);
@@ -93,6 +97,7 @@
 	// Base navigation items for Chat (settings moved to user dropdown)
 	const baseNavItems: PillNavItem[] = [
 		{ href: '/chat', label: 'Chat', icon: 'home' },
+		{ href: '/compare', label: 'Vergleichen', icon: 'scale' },
 		{ href: '/templates', label: 'Templates', icon: 'document' },
 		{ href: '/spaces', label: 'Spaces', icon: 'building' },
 		{ href: '/documents', label: 'Dokumente', icon: 'archive' },
@@ -183,6 +188,11 @@
 
 		await authStore.initialize();
 
+		// Show guest welcome modal for unauthenticated users
+		if (!authStore.isAuthenticated && shouldShowGuestWelcome('chat')) {
+			showGuestWelcome = true;
+		}
+
 		// Load user settings if authenticated
 		if (authStore.isAuthenticated) {
 			await userSettings.load();
@@ -291,6 +301,23 @@
 		conversationCount={sessionConversationCount}
 		onClose={() => (showAuthGateModal = false)}
 	/>
+
+	<!-- Guest Welcome Modal -->
+	<GuestWelcomeModal
+		appId="chat"
+		visible={showGuestWelcome}
+		onClose={() => (showGuestWelcome = false)}
+		onLogin={() => {
+			showGuestWelcome = false;
+			goto('/login');
+		}}
+		onRegister={() => {
+			showGuestWelcome = false;
+			goto('/register');
+		}}
+		helpHref="/help"
+		locale={currentLocale === 'en' ? 'en' : 'de'}
+	/>
 {/if}
 
 <style>