refactor(notification): provide notification store via context + fixture

Converts the module-singleton notificationStore into a context-provided
store so its specs can drive it without mocking the module. notifications.svelte
now exports createNotificationStore() (the former singleton body), plus
provideNotificationStore()/getNotificationStore()/NOTIFICATION_KEY mirroring
the confirm service. Root +layout provides it; NotificationBell and the
Chronik page read it via getNotificationStore().

Tests:
- notifications.svelte.spec drives a fresh createNotificationStore() per test
  (replacing __resetForTest/__setNavigateForTest with setNavigate()).
- notification.test-fixture.svelte wraps the bell, provides the store, and
  exposes setNotifications(items) via onReady (option b).
- NotificationBell.svelte.spec asserts the announced unread count across the
  empty / single / many / error a11y states (AC#5), stubbing EventSource+fetch.
- aktivitaeten page spec injects a real store via render context.

Per the recorded Phase-2b decision (full context refactor). Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/notification/NotificationBell.svelte

@@ -2,13 +2,13 @@
 import { onMount, onDestroy } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
 import { clickOutside } from '$lib/shared/actions/clickOutside';
-import { notificationStore } from '$lib/notification/notifications.svelte';
+import { getNotificationStore } from '$lib/notification/notifications.svelte';
 import NotificationDropdown from './NotificationDropdown.svelte';
 
 let open = $state(false);
 let bellButtonEl: HTMLButtonElement | null = null;
 
-const stream = notificationStore;
+const stream = getNotificationStore();
 
 async function toggleDropdown() {
 	open = !open;