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>
