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/store/__tests__/pluginStore.test.ts

@@ -25,6 +25,7 @@ const mockManifests: PluginUiManifest[] = [
     dashboard_widgets: [
       { id: 'widget_a', label_key: 'widgets.a', label: 'Widget A', component: '@/components/WidgetA', icon: 'A', order: 200, col_span: 2, row_span: 1, permission: '' },
     ],
+    custom_fields: [],
   },
   {
     name: 'plugin_b',
@@ -46,6 +47,7 @@ const mockManifests: PluginUiManifest[] = [
     dashboard_widgets: [
       { id: 'widget_b', label_key: 'widgets.b', label: 'Widget B', component: '@/components/WidgetB', icon: 'B', order: 100, col_span: 1, row_span: 1, permission: '' },
     ],
+    custom_fields: [],
   },
 ];