refactor(packing): share the packing list logic between desktop and phone

usePackingItemActions, usePackingTemplates and usePackingImport take the
item, template and bulk import logic out of usePackingListPanel and
MPackingListTab, and the phone listsModel keeps only its priority
tokens.

When adding to a category the desktop looks for the placeholder in the
store and the phone in its rendered items. The desktop toasts on an
empty import, the phone runs one import at a time. The effects run in
the same order as before.

client/src/components/Packing/usePackingCategoryAssignees.ts

@@ -0,0 +1,44 @@
+import { useEffect, useState } from 'react';
+
+import { packingApi } from '../../api/client';
+
+type Translate = (key: string, params?: Record<string, string | number>) => string;
+interface Toaster {
+  error: (message: string) => void;
+}
+
+/** Who looks after a packing category, as the server lists them. */
+export interface CategoryAssignee {
+  user_id: number;
+  username: string;
+  avatar?: string | null;
+  is_guest?: boolean;
+}
+
+/**
+ * Who looks after each packing category, for both packing lists: loaded per
+ * trip, and replaced category by category. A failed load leaves nobody assigned.
+ */
+export function usePackingCategoryAssignees({ tripId, t, toast }: { tripId: number; t: Translate; toast: Toaster }) {
+  const [categoryAssignees, setCategoryAssignees] = useState<Record<string, CategoryAssignee[]>>({});
+
+  useEffect(() => {
+    packingApi
+      .getCategoryAssignees(tripId)
+      .then((data) => setCategoryAssignees(data.assignees || {}))
+      .catch(() => {
+        // A failed read leaves every category unassigned; there is nothing to undo.
+      });
+  }, [tripId]);
+
+  const setAssignees = async (category: string, userIds: number[]) => {
+    try {
+      const data = await packingApi.setCategoryAssignees(tripId, category, userIds);
+      setCategoryAssignees((prev) => ({ ...prev, [category]: data.assignees || [] }));
+    } catch {
+      toast.error(t('packing.toast.saveError'));
+    }
+  };
+
+  return { categoryAssignees, setAssignees };
+}