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/notification.test-fixture.svelte

@@ -0,0 +1,13 @@
+<script lang="ts">
+import { provideNotificationStore, type NotificationItem } from './notifications.svelte';
+import NotificationBell from './NotificationBell.svelte';
+
+type Api = { setNotifications: (items: NotificationItem[]) => void };
+
+let { onReady }: { onReady: (api: Api) => void } = $props();
+
+const store = provideNotificationStore();
+onReady({ setNotifications: store.setNotifications });
+</script>
+
+<NotificationBell />