feat(5.20): Custom Fields — plugin-defined fields in Contact UI

- Add CustomFieldDefinition to PluginManifest (text/number/date/select/multiselect/boolean)
- Add GET/PATCH /api/v1/contacts/{id}/custom-fields routes
- Merge plugin definitions with stored values in contacts.custom JSONB
- Add CustomFieldRenderer component (read + edit modes)
- Integrate into ContactDetail (read-only) and ContactEditModal (editable)
- Add custom_fields to pluginStore and active manifests API
- Add useCustomFields/useUpdateCustomFields React Query hooks
- Tests: test_custom_fields.py (9 tests) + CustomFieldRenderer.test.tsx (5 tests)
- i18n keys already present (contacts.customFields)

frontend/src/api/customFields.ts

@@ -0,0 +1,42 @@
+/**
+ * Custom fields API hooks — merge plugin definitions with stored values.
+ */
+
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { apiGet, apiPatch } from './client';
+
+export interface CustomFieldDefinition {
+  name: string;
+  label: string;
+  label_key: string;
+  field_type: 'text' | 'number' | 'date' | 'select' | 'multiselect' | 'boolean';
+  options: string[];
+  default_value: any;
+  required: boolean;
+  entity: string;
+  plugin: string;
+  value: any;
+}
+
+export interface CustomFieldsResponse {
+  fields: CustomFieldDefinition[];
+}
+
+export function useCustomFields(contactId?: string) {
+  return useQuery({
+    queryKey: ['custom-fields', contactId],
+    queryFn: () => apiGet<CustomFieldsResponse>(`/contacts/${contactId}/custom-fields`),
+    enabled: !!contactId,
+  });
+}
+
+export function useUpdateCustomFields() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ contactId, values }: { contactId: string; values: Record<string, any> }) =>
+      apiPatch<CustomFieldsResponse>(`/contacts/${contactId}/custom-fields`, { values }),
+    onSuccess: (_data, variables) => {
+      queryClient.invalidateQueries({ queryKey: ['custom-fields', variables.contactId] });
+    },
+  });
+}