refactor(notifications): convert per-component stream hook to module-level singleton

Replaces the per-component createNotificationStream() factory with a shared
$lib/stores/notifications.svelte.ts singleton. Ref-counted init()/destroy()
ensures one EventSource per tab no matter how many consumers mount
simultaneously.

Motivation: the /chronik "Für dich" box (#285) needs the same live-arrival
stream that NotificationBell already consumes. Two factories would open two
SSE connections per tab — this refactor avoids the silent regression before
it ships.

- New: src/lib/stores/notifications.svelte.ts (module state, refcount)
- New: src/lib/stores/notifications.svelte.spec.ts (proves single EventSource
  across multiple consumers + ref-counted teardown)
- Deleted: src/lib/hooks/useNotificationStream.svelte.ts (factory)
- Deleted: src/lib/hooks/__tests__/useNotificationStream.svelte.test.ts
- NotificationBell now imports the singleton

Part of #285.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/NotificationBell.svelte

@@ -3,13 +3,13 @@ import { onMount, onDestroy } from 'svelte';
 import { goto } from '$app/navigation';
 import { m } from '$lib/paraglide/messages.js';
 import { clickOutside } from '$lib/actions/clickOutside';
-import { createNotificationStream } from '$lib/hooks/useNotificationStream.svelte';
+import { notificationStore } from '$lib/stores/notifications.svelte';
 import NotificationDropdown from './NotificationDropdown.svelte';
 
 let open = $state(false);
 let bellButtonEl: HTMLButtonElement | null = null;
 
-const stream = createNotificationStream();
+const stream = notificationStore;
 
 async function toggleDropdown() {
 	open = !open;