feat(app): event status transitions on detail header

Add transition buttons from allowed_transitions with Dutch labels,
confirmation dialog, TanStack mutation + cache invalidation, and
422/generic error handling via notification store.

Made-with: Cursor

apps/app/src/lib/event-status.ts

@@ -0,0 +1,39 @@
+import type { EventStatus } from '@/types/event'
+
+const STATUS_ORDER: Record<EventStatus, number> = {
+  draft: 0,
+  published: 1,
+  registration_open: 2,
+  buildup: 3,
+  showday: 4,
+  teardown: 5,
+  closed: 6,
+}
+
+export const EVENT_STATUS_LABEL_NL: Record<EventStatus, string> = {
+  draft: 'Concept',
+  published: 'Gepubliceerd',
+  registration_open: 'Registratie open',
+  buildup: 'Opbouw',
+  showday: 'Showdag',
+  teardown: 'Afbouw',
+  closed: 'Afgesloten',
+}
+
+export function eventStatusLabelNl(status: EventStatus): string {
+  return EVENT_STATUS_LABEL_NL[status]
+}
+
+/** UI styling: forward in lifecycle, rollback, or closing the event. */
+export function transitionVisualKind(
+  currentStatus: EventStatus,
+  targetStatus: EventStatus,
+): 'forward' | 'backward' | 'dangerous' {
+  if (targetStatus === 'closed')
+    return 'dangerous'
+
+  const current = STATUS_ORDER[currentStatus]
+  const next = STATUS_ORDER[targetStatus]
+
+  return next < current ? 'backward' : 'forward'
+}