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

docs/backend/forms.md

@@ -150,6 +150,8 @@ fields:
 
 The admin SPA fetches the options from `GET <prefix>/api/v1/{vendor}/{plugin}/{controller}/fields/{field}/dependent-options?values[<sibling>]=<value>` whenever the sibling changes; for a dropdown inside a repeater row, `{field}` is `<repeater>.<field>` and the provider receives that path as the field name. The route is guarded like the form schema (a backend session with the controller's permissions) and answers `{applies, options: [{value, label, depth}]}` with labels in the request locale.
 
+In the admin, the dropdown stays disabled with a hint until the sibling has a value, while its options load, and when the sibling's value takes no choice. A failed load offers a retry. Answers are cached per sibling value while the form is open. When the administrator changes the sibling and the current choice is not among the new options, the dropdown clears it, the form counts as changed and a screen reader hears that the field was cleared. A stored value that is missing from the options on load is never replaced by another choice: the dropdown shows its placeholder and the save answers with the validation message below.
+
 The server checks the value again on every save, so a client cannot store a reference the options do not offer. When the dropdown or its sibling is written, the provider is asked with the sibling's submitted value (or the stored one when only the dropdown is sent), and in every row of a written repeater with the row's own sibling value. A value that does not apply is stored as null; a value that is not among the options is a 422 with `form.dependent_invalid` ("Choose one of the listed options.") on the field, or under `<repeater>.<index>.<field>` in a row. Null passes this check; `required` still applies.
 
 Menu item types are the common source of such options. A menu or sitemap controller answers with `signpost.ReferenceOptions`, which turns the chosen type's reference tree into options with their depth and answers `Applies` false for a type without references:
@@ -425,6 +427,8 @@ fields:
 
 A save body sends the list of rows. Null is stored as an empty list; anything that is not a list of objects is a 422 on the field. Each row keeps only the nested field names, so extra keys are dropped, and its values are coerced per nested type: a number to a JSON number, a checkbox or switch to a boolean, everything else to a string or null. A required nested value missing from a row is a 422 under `<field>.<index>.<nested>`, such as `content.3.answer`. Too many or too few rows is a 422 on the field (`repeater.max_items`, `repeater.min_items`), and a required repeater needs at least one row. Every nested `richeditor` value is sanitized like a top-level one, with its notices under the row path, and every nested dependent dropdown is rechecked against its row's sibling.
 
+In the admin, every row has a drag handle, its number, a title (the `titleFrom` value, or "Item N"), buttons to move it up or down, collapse it and remove it, and the nested fields below. The handle also moves the row with the arrow keys. Removing a row that holds a value asks first; an empty row goes at once. The add button appends a row with the nested defaults, and a counter shows the number of rows when `maxItems` is set. Every change only edits the form: nothing is sent until the record is saved, and Cancel discards it. A validation error under a row path opens that row, marks it and focuses the first invalid field.
+
 The repeater is the one field type whose value may be a nested list: `cabana.ProjectWritableFields` lets a repeater's list through and still drops nested values on every other field. The type is supported in controller forms only; settings forms and relation forms refuse it at start-up.
 
 ## Rich text