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/profile/+page.server.ts

@@ -1,10 +1,15 @@
 import { fail } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
 import type { PageServerLoad, Actions } from './$types';
 import { createApiClient } from '$lib/api.server';
 import { getErrorMessage } from '$lib/errors';
 
-export const load: PageServerLoad = async ({ locals }) => {
-	return { user: locals.user };
+const apiBase = () => env.API_INTERNAL_URL || 'http://localhost:8080';
+
+export const load: PageServerLoad = async ({ locals, fetch }) => {
+	const res = await fetch(`${apiBase()}/api/users/me/notification-preferences`);
+	const notificationPrefs = res.ok ? await res.json() : null;
+	return { user: locals.user, notificationPrefs };
 };
 
 export const actions: Actions = {
@@ -50,5 +55,26 @@ export const actions: Actions = {
 		}
 
 		return { passwordSuccess: true };
+	},
+
+	updateNotificationPrefs: async ({ request, fetch }) => {
+		const formData = await request.formData();
+		const body = {
+			notifyOnReply: formData.get('notifyOnReply') === 'true',
+			notifyOnMention: formData.get('notifyOnMention') === 'true'
+		};
+
+		const res = await fetch(`${apiBase()}/api/users/me/notification-preferences`, {
+			method: 'PUT',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify(body)
+		});
+
+		if (!res.ok) {
+			const data = await res.json().catch(() => ({}));
+			return fail(res.status, { prefsError: getErrorMessage(data?.code) });
+		}
+
+		return { prefsSuccess: true };
 	}
 };