fix(compiler): migrate custom-fields-manager to useQuery

set-state-in-effect: 44 → 43.

Eight admin list/load sites migrated total this session; the
remaining ~43 hits are predominantly the dialog/form open→reset
pattern (intentional setState-in-effect when a dialog opens to
populate fields from props). Cleanest fix is key-based remount
of the dialog body; tracked in BACKLOG as a focused refactor pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/custom-fields/custom-fields-manager.tsx

@@ -1,6 +1,7 @@
 'use client';
 
-import { useState, useCallback, useEffect } from 'react';
+import { useState } from 'react';
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import { type ColumnDef } from '@tanstack/react-table';
 import { Pencil, Trash2, Plus } from 'lucide-react';
 
@@ -33,27 +34,29 @@ const FIELD_TYPE_LABELS: Record<string, string> = {
 
 // ─── Component ────────────────────────────────────────────────────────────────
 
+const FIELDS_QUERY_KEY = ['admin', 'custom-fields'] as const;
+
 export function CustomFieldsManager() {
+  const queryClient = useQueryClient();
   const [activeTab, setActiveTab] = useState<EntityTab>('client');
-  const [fields, setFields] = useState<CustomFieldDefinition[]>([]);
-  const [loading, setLoading] = useState(true);
   const [formOpen, setFormOpen] = useState(false);
   const [editingField, setEditingField] = useState<CustomFieldDefinition | null>(null);
-  const [deletingId, setDeletingId] = useState<string | null>(null);
 
-  const fetchFields = useCallback(async () => {
-    setLoading(true);
-    try {
-      const res = await apiFetch<{ data: CustomFieldDefinition[] }>('/api/v1/admin/custom-fields');
-      setFields(res.data);
-    } finally {
-      setLoading(false);
-    }
-  }, []);
+  const { data: fields = [], isLoading: loading } = useQuery<CustomFieldDefinition[]>({
+    queryKey: FIELDS_QUERY_KEY,
+    queryFn: () =>
+      apiFetch<{ data: CustomFieldDefinition[] }>('/api/v1/admin/custom-fields').then(
+        (r) => r.data,
+      ),
+  });
 
-  useEffect(() => {
-    void fetchFields();
-  }, [fetchFields]);
+  const fetchFields = () => queryClient.invalidateQueries({ queryKey: FIELDS_QUERY_KEY });
+
+  const deleteMutation = useMutation({
+    mutationFn: (id: string) => apiFetch(`/api/v1/admin/custom-fields/${id}`, { method: 'DELETE' }),
+    onSuccess: () => fetchFields(),
+  });
+  const deletingId = deleteMutation.isPending ? deleteMutation.variables : null;
 
   const filteredFields = fields.filter((f) => f.entityType === activeTab);
 
@@ -67,14 +70,8 @@ export function CustomFieldsManager() {
     setFormOpen(true);
   }
 
-  async function handleDelete(id: string) {
-    setDeletingId(id);
-    try {
-      await apiFetch(`/api/v1/admin/custom-fields/${id}`, { method: 'DELETE' });
-      await fetchFields();
-    } finally {
-      setDeletingId(null);
-    }
+  function handleDelete(id: string) {
+    deleteMutation.mutate(id);
   }
 
   function getDeleteDescription(field: CustomFieldDefinition): string {