fix(calendar): add timezone fallback in test mock to match Calendar type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/mana/apps/web/src/lib/modules/calendar/stores/calendars.test.ts

@@ -77,7 +77,7 @@ describe('calendarsStore.setAsDefault', () => {
 			isDefault: c.isDefault as boolean,
 			isVisible: c.isVisible as boolean,
 			color: c.color as string,
-			timezone: (c.timezone as string) ?? null,
+			timezone: (c.timezone as string) ?? 'Europe/Berlin',
 			createdAt: (c.createdAt as string) ?? '',
 			updatedAt: (c.updatedAt as string) ?? '',
 		}));