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/CustomSelect.tsx

@@ -12,9 +12,10 @@ interface Props {
   options: SelectOption[];
   placeholder: string;
   onChange: (value: string) => void;
+  keepOpen?: boolean;
 }
 
-export default function CustomSelect({ options, placeholder, onChange }: Props) {
+export default function CustomSelect({ options, placeholder, onChange, keepOpen = false }: Props) {
   const [open, setOpen] = useState(false);
   const ref = useRef<HTMLDivElement>(null);
 
@@ -29,7 +30,7 @@ export default function CustomSelect({ options, placeholder, onChange }: Props)
   }, [open]);
 
   function select(value: string) {
-    setOpen(false);
+    if (!keepOpen) setOpen(false);
     onChange(value);
   }
 
@@ -62,6 +63,15 @@ export default function CustomSelect({ options, placeholder, onChange }: Props)
               {opt.label}
             </button>
           ))}
+          {keepOpen && (
+            <button
+              type="button"
+              onClick={() => setOpen(false)}
+              className="w-full px-4 py-3 text-base font-medium text-center bg-gray-100 hover:bg-gray-200 text-gray-700 rounded-b-lg border-t-2 border-gray-300"
+            >
+              Fertig
+            </button>
+          )}
         </div>
       )}
     </div>