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/ui/ExtensionFailure.vue

@@ -0,0 +1,22 @@
+<script setup lang="ts">
+import { CircleAlert } from '@lucide/vue'
+
+// The extension failure box (UI-SPEC S5): UnsupportedField's geometry with a
+// danger icon, shown when a widget or partial could not be loaded. One line
+// sits centred in the 44px box; longer text wraps, the box grows with 10px
+// vertical padding and the icon stays on the first line.
+defineProps<{ text: string }>()
+</script>
+
+<template>
+  <div
+    role="alert"
+    data-extension-failure
+    class="flex min-h-input items-center rounded-control border-[1.5px] border-dashed border-border-strong bg-subtle px-3.5 py-2.5 text-[13px] text-muted"
+  >
+    <span class="flex min-w-0 items-start gap-2.5">
+      <CircleAlert :size="16" class="mt-0.5 shrink-0 text-danger" aria-hidden="true" />
+      <span class="min-w-0 [overflow-wrap:anywhere]">{{ text }}</span>
+    </span>
+  </div>
+</template>