feat(#71): add notification bell + preferences UI

- NotificationBell.svelte: bell icon in header with unread badge, dropdown showing last 10 notifications, mark-all-read, click-outside close, keyboard Escape support, polls every PUBLIC_NOTIFICATION_POLL_MS ms
- Wire NotificationBell into +layout.svelte between ThemeToggle and UserMenu (authenticated users only)
- Profile page: add notification preferences card with notifyOnReply / notifyOnMention toggles, loaded via GET and saved via PUT /api/users/me/notification-preferences
- i18n: de/en/es message keys for bell, notifications list, and preference labels

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -4,6 +4,7 @@ import { page } from '$app/state';
 import { onMount } from 'svelte';
 import LanguageSwitcher from '$lib/components/LanguageSwitcher.svelte';
 import ThemeToggle from '$lib/components/ThemeToggle.svelte';
+import NotificationBell from '$lib/components/NotificationBell.svelte';
 import AppNav from './AppNav.svelte';
 import UserMenu from './UserMenu.svelte';
 
@@ -52,6 +53,11 @@ const userInitials = $derived.by(() => {
 						<!-- Theme toggle -->
 						<ThemeToggle />
 
+						<!-- Notification bell (authenticated users only) -->
+						{#if data?.user}
+							<NotificationBell />
+						{/if}
+
 						<!-- User menu -->
 						<UserMenu userInitials={userInitials} />
 					</div>