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/components/contacts/ContactDetail.tsx

@@ -14,6 +14,8 @@ import { usePluginStore } from '@/store/pluginStore';
 import { useAIUIControlStore } from '@/store/aiUIControlStore';
 import { PluginPage } from '@/components/plugins/PluginLoader';
 import { useAuthStore } from '@/store/authStore';
+import { CustomFieldRenderer } from '@/components/contacts/CustomFieldRenderer';
+import { useCustomFields } from '@/api/customFields';
 import {
   type UnifiedContact,
   type ContactPerson,
@@ -177,6 +179,10 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   }, [aiActiveModal]);
   const user = useAuthStore(s => s.user);
 
+  // Custom fields from plugin definitions
+  const customFieldDefs = usePluginStore(s => s.getCustomFieldsForEntity('contact'));
+  const { data: customFieldsData } = useCustomFields(contact?.id);
+
   if (loading) {
     return (
       <div className="flex items-center justify-center py-12" data-testid="contact-detail-loading">
@@ -420,11 +426,12 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
         </Section>
 
         {/* Custom Fields */}
-        {contact.custom && Object.keys(contact.custom).length > 0 && (
+        {contact.id && customFieldDefs.length > 0 && (
           <Section title={t('contacts.customFields')}>
-            <pre className="text-xs text-secondary-700 bg-secondary-50 rounded p-2 overflow-x-auto">
-              {JSON.stringify(contact.custom, null, 2)}
-            </pre>
+            <CustomFieldRenderer
+              fields={customFieldsData?.fields || []}
+              mode="read"
+            />
           </Section>
         )}