Add multi-select mode to BeoSelector and ObjektSelector

CustomSelect gains keepOpen prop: dropdown stays open after each
selection, closes via Fertig button or tap outside. ObjektSelector
separates Neu into its own button to keep the dropdown flow clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/BeoSelector.tsx

@@ -54,9 +54,10 @@ export default function BeoSelector({ selected, onChange }: Props) {
 
       {available.length > 0 && (
         <CustomSelect
-          placeholder="+ BEO hinzufügen"
+          placeholder="+ BEOs hinzufügen"
           options={available.map((b) => ({ value: String(b.ID), label: `${b.Kuerzel} — ${b.Name}` }))}
           onChange={add}
+          keepOpen
         />
       )}
     </div>