feat(17-02): repeater field and dependent dropdown states

- RepeaterField: rows with drag handle, index, titleFrom title, error chip,
  move up/down (buttons, ArrowUp/ArrowDown on the handle, native drag and
  drop), expand/collapse, remove (asks only for a row holding a value),
  add with nested defaults, maxItems counter, empty state, collapse all;
  every change edits form state only
- 422 row paths (content.3.answer) open the row, mark it and focus the
  nested control; the banner no longer repeats row messages
- DropdownField dependsOn: waiting, loading, not used, empty, loaded with
  depth indent, failed with retry; cached per sibling value, last request
  wins, a stale value is cleared after a sibling change and announced
- FormView provides form feedback (errors and notices by row path, the
  polite live region, its confirm dialog) and the options cache; FormGrid
  provides the sibling scope for repeater rows
- repeater.* and form.dependent_* phrase keys (en, pl), smoke specs, docs

admin/src/components/form/PreviewField.vue

@@ -21,7 +21,7 @@ const props = defineProps<{
 }>()
 
 /** Types rendered by their own control in its read-only mode. */
-const ownControl = new Set(['datepicker', 'fileupload', 'partial', 'permissioneditor', 'richeditor'])
+const ownControl = new Set(['datepicker', 'fileupload', 'partial', 'permissioneditor', 'richeditor', 'repeater'])
 
 const controlId = computed(() => `${props.idPrefix ?? 'preview'}-${props.field.name}`)
 const labelId = computed(() => `${controlId.value}-label`)