feat(accounting): event booking via dropdown (Company or an event)

Replaces the Company/Event toggle + numeric Event-ID input with a single
EventBookingSelect dropdown (Company = null, else a specific event, fetched via
eventsService). Used by both the incoming-invoice triage and the expense add
form. Projects stay a separate aggregation of events and are intentionally not
a booking target here.

Verified: tsc --noEmit clean; npm run build green.

frontend/src/components/admin/EventBookingSelect.tsx

@@ -0,0 +1,39 @@
+/**
+ * Booking-target dropdown for accounting (expenses + incoming invoices).
+ * "Company" (value null) or a specific event. Projects remain a separate
+ * aggregation of events and are intentionally NOT a booking target here.
+ */
+import React from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { useTranslation } from 'react-i18next';
+import { eventsService } from '../../services/events.service';
+
+interface Props {
+  value: number | null;
+  onChange: (eventId: number | null) => void;
+  className?: string;
+}
+
+export const EventBookingSelect: React.FC<Props> = ({ value, onChange, className }) => {
+  const { t } = useTranslation();
+  const { data } = useQuery({
+    queryKey: ['events-for-booking'],
+    queryFn: () => eventsService.getEvents(1, 200),
+    staleTime: 60_000,
+  });
+  const events = data?.events ?? [];
+  const cls = className
+    || 'w-full rounded-md border border-neutral-300 dark:border-neutral-600 bg-white dark:bg-neutral-800 px-3 py-2 text-sm';
+
+  return (
+    <select className={cls} value={value == null ? '' : String(value)}
+      onChange={(e) => onChange(e.target.value ? Number(e.target.value) : null)}>
+      <option value="">{t('accounting.booking.company', 'Company')}</option>
+      {events.map((ev) => (
+        <option key={ev.id} value={ev.id}>{ev.event_name}</option>
+      ))}
+    </select>
+  );
+};
+
+export default EventBookingSelect;