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/formContext.ts

@@ -0,0 +1,28 @@
+// What a record form shares with extension controls (D-05, D-08). FormView
+// provides these; a widget reads the current values and the locale, and
+// writes action results back through patch, which marks the form dirty and
+// clears that field's errors exactly like typing into it. A form without a
+// provider (settings pages) gets the defaults of the injecting control.
+import type { InjectionKey, Ref } from 'vue'
+import type { AdminRecord } from '../../api/types'
+
+/** Read-only view of the form's current values. */
+export const FORM_VALUES: InjectionKey<Readonly<Ref<AdminRecord>>> = Symbol('summer.form.values')
+
+/** Sets one field's value as if the admin had edited it. Nothing is saved. */
+export const FORM_PATCH: InjectionKey<(name: string, value: unknown) => void> = Symbol('summer.form.patch')
+
+/** The form schema's locale (meta.locale). */
+export const FORM_LOCALE: InjectionKey<Readonly<Ref<string>>> = Symbol('summer.form.locale')
+
+/**
+ * The form controller's plugin assets: resolves once its scripts have loaded
+ * or failed, with the failed URLs (pluginAssets.loadControllerAssets).
+ */
+export const FORM_ASSETS: InjectionKey<() => Promise<string[]>> = Symbol('summer.form.assets')
+
+/** Event a widget element dispatches to run its action (bubbles, composed). */
+export const WIDGET_EVENT = 'summer-action'
+
+/** How long a widget waits for its custom element to be defined. */
+export const WIDGET_TIMEOUT = 5000