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/tests/fixtures/extension.form-schema.json

@@ -0,0 +1,68 @@
+{
+  "data": {
+    "name": "Widget",
+    "fields": [
+      {
+        "name": "name",
+        "label": "Name",
+        "type": "text",
+        "span": "left",
+        "required": true
+      },
+      {
+        "name": "color",
+        "label": "Colour",
+        "type": "text",
+        "span": "right"
+      },
+      {
+        "name": "lookup",
+        "label": "Catalogue",
+        "type": "widget",
+        "comment": "Fills in the name. Save to keep it.",
+        "widget": "acme-demo-lookup",
+        "action": "lookup",
+        "actionLabel": "Look up",
+        "fill": ["name"]
+      }
+    ],
+    "messages": {
+      "create": {
+        "other": "New widget"
+      },
+      "update": {
+        "other": "Editing widget"
+      },
+      "saved": {
+        "other": "Widget „:name” saved"
+      },
+      "deleteConfirm": {
+        "other": "Delete this widget?"
+      },
+      "deleted": {
+        "other": "Widget deleted"
+      }
+    },
+    "meta": {
+      "locale": "en"
+    },
+    "assets": {
+      "scripts": ["/admin-test/assets/acme/demo/js/lookup.js?v=abc"],
+      "styles": []
+    },
+    "redirects": {
+      "create": {
+        "redirect": "acme/demo/widgets/update/:id",
+        "redirectClose": "acme/demo/widgets"
+      },
+      "update": {
+        "redirect": "acme/demo/widgets",
+        "redirectClose": "acme/demo/widgets"
+      },
+      "default": "acme/demo/widgets"
+    }
+  },
+  "meta": {
+    "locale": "en"
+  }
+}

admin/tests/fixtures/lang.json

@@ -21,6 +21,18 @@
     "backend::lang.auth.title": {
       "other": "Witaj ponownie"
     },
+    "backend::lang.extension.action_failed": {
+      "other": "Nie udało się wykonać akcji. Spróbuj ponownie."
+    },
+    "backend::lang.extension.busy": {
+      "other": "Wczytywanie…"
+    },
+    "backend::lang.extension.partial_failed": {
+      "other": "Nie udało się wczytać tej sekcji. Odśwież stronę, aby spróbować ponownie."
+    },
+    "backend::lang.extension.widget_failed": {
+      "other": "Nie udało się wczytać tej kontrolki. Odśwież stronę, a jeśli błąd wróci, sprawdź zasoby wtyczki."
+    },
     "backend::lang.form.add": {
       "other": "Dodaj"
     },

admin/tests/fixtures/typed.ts

@@ -7,6 +7,7 @@ import type { components } from '../../src/api/schema'
 import langJson from './lang.json'
 import navigationJson from './navigation.json'
 import settingsJson from './settings.json'
+import extensionFormSchemaJson from './extension.form-schema.json'
 import formSchemaJson from './widgets.form-schema.json'
 import listJson from './widgets.list.json'
 import listSchemaJson from './widgets.list-schema.json'
@@ -27,6 +28,8 @@ export const settingsFixture: {
   values: S['cabana.Envelope-cabana_SettingsResult']
 } = settingsJson
 export const formSchemaFixture: S['cabana.Envelope-cabana_FormView'] = formSchemaJson
+/** A form with a plugin widget and its controller script (Phase 10.1). */
+export const extensionFormSchemaFixture: S['cabana.Envelope-cabana_FormView'] = extensionFormSchemaJson
 export const listFixture: Rows = listJson
 export const listSchemaFixture: S['cabana.Envelope-cabana_ListSchema'] = listSchemaJson
 export const optionsFixture: {