refactor(mui): migrate ReservationPicker to kit Select

Replace the raw admin-form-select with the MUI kit Select (string-based
value, converted at the boundary). Same query (warehouseReservationList,
ACTIVE, perPage 100), same option labels, same onChange contract
(reservationId|null, remainingQty).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/warehouse/ReservationPicker.tsx

@@ -1,4 +1,6 @@
 import { useQuery } from "@tanstack/react-query";
+import MenuItem from "@mui/material/MenuItem";
+import { Select } from "../../ui";
 import { warehouseReservationListOptions } from "../../lib/queries/warehouse";
 
 interface ReservationPickerProps {
@@ -26,11 +28,9 @@ export default function ReservationPicker({
   const reservations = result?.data ?? [];
 
   return (
-    <select
-      className="admin-form-select"
-      value={value ?? ""}
-      onChange={(e) => {
-        const val = e.target.value;
+    <Select
+      value={value == null ? "" : String(value)}
+      onChange={(val) => {
         if (!val) {
           onChange(null, 0);
           return;
@@ -42,13 +42,13 @@ export default function ReservationPicker({
         }
       }}
     >
-      <option value="">— žádná rezervace —</option>
+      <MenuItem value="">— žádná rezervace —</MenuItem>
       {reservations.map((r) => (
-        <option key={r.id} value={r.id}>
+        <MenuItem key={r.id} value={String(r.id)}>
           R{r.id} — {r.project?.name ?? `Projekt #${r.project_id}`} — zbývá:{" "}
           {Number(r.remaining_qty)} {r.item?.unit ?? "ks"}
-        </option>
+        </MenuItem>
       ))}
-    </select>
+    </Select>
   );
 }