feat(10.1-02): mount plugin widget elements and run their actions from the form

- pluginAssets loads controller scripts and stylesheets from {base}/assets/ only, once per URL
- WidgetField mounts the custom element with attributes only and posts summer-action through the typed client
- Only declared fill keys returned by the server are patched; the form turns dirty and nothing saves
- widget is a registered valueless type rendered on create and update, labelled as a group
- backend::lang.extension strings in en and pl; embedded dist rebuilt

admin/src/components/form/registry.ts

@@ -3,7 +3,9 @@
 // dashed box, so an unknown type never breaks the form. The relation manager
 // is registered like any control but holds no form value: it edits its
 // relation through its own endpoints and renders only on an existing record.
-// Phase 10.1 turns this seam into the plugin extension point.
+// Phase 10.1 adds the plugin extension types: a widget mounts a plugin custom
+// element (D-04, D-09). It holds no form value either, but it renders on
+// create and update.
 import type { Component } from 'vue'
 import CheckboxField from './fields/CheckboxField.vue'
 import DropdownField from './fields/DropdownField.vue'
@@ -14,6 +16,7 @@ import SwitchField from './fields/SwitchField.vue'
 import TextField from './fields/TextField.vue'
 import TextareaField from './fields/TextareaField.vue'
 import UnsupportedField from './fields/UnsupportedField.vue'
+import WidgetField from './fields/WidgetField.vue'
 
 // The control helpers live in ./control so the field components never import
 // this module: registry -> field -> registry would be an import cycle whose
@@ -33,24 +36,34 @@ const renderers = new Map<string, Component>([
   ['checkbox', CheckboxField],
   ['relation', RelationField],
   [RELATION_MANAGER, RelationManager],
+  ['widget', WidgetField],
 ])
 
 /** Types whose control shows the label itself (toggle cards, relation manager). */
 const selfLabelled = new Set<string>(['switch', 'checkbox', RELATION_MANAGER])
 
 /**
- * Types that need a saved record and hold no form value: never rendered on
- * create, never part of the save body (D-05, design screen 5).
+ * Types that need a saved record: never rendered on create (D-05, design
+ * screen 5).
  */
 const recordBound = new Set<string>([RELATION_MANAGER])
 
+/** Types that hold no form value: never part of the save body (D-09). */
+const valueless = new Set<string>([RELATION_MANAGER, 'widget'])
+
+/**
+ * Types whose control is a group rather than one focusable element: the
+ * visible label is a span the group points at, not a label for an input.
+ */
+const groupLabelledTypes = new Set<string>(['widget'])
+
 export function rendererFor(type: string): Component {
   return renderers.get(type) ?? UnsupportedField
 }
 
 /** Whether the SPA can edit values of this field type. */
 export function isRegistered(type: string): boolean {
-  return renderers.has(type) && !recordBound.has(type)
+  return renderers.has(type) && !valueless.has(type)
 }
 
 /** Whether a field type renders only on an existing record (relation manager). */
@@ -61,3 +74,8 @@ export function needsRecord(type: string): boolean {
 export function ownsLabel(type: string): boolean {
   return selfLabelled.has(type)
 }
+
+/** Whether a field type's label labels a group (widget) instead of a control. */
+export function groupLabelled(type: string): boolean {
+  return groupLabelledTypes.has(type)
+}