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/formState.ts

@@ -296,3 +296,65 @@ export function focusField(prefix: string, name: string): boolean {
   element.focus()
   return true
 }
+
+/** The top-level field of an error or notice key: `content` for `content.3.answer`. */
+export function keyField(key: string): string {
+  const dot = key.indexOf('.')
+  return dot < 0 ? key : key.slice(0, dot)
+}
+
+/** Whether a field has errors on itself or, for a repeater, on any row path. */
+export function fieldHasErrors(errors: Record<string, string[]>, name: string): boolean {
+  return Object.keys(errors).some((key) => key === name || key.startsWith(`${name}.`))
+}
+
+/** The entries of one repeater row (`<field>.<index>.<nested>`) keyed by nested name. */
+export function rowEntries(map: Record<string, string[]>, field: string, index: number): Record<string, string[]> {
+  const prefix = `${field}.${index}.`
+  const out: Record<string, string[]> = {}
+  for (const [key, lines] of Object.entries(map)) {
+    if (key.startsWith(prefix) && key.length > prefix.length) {
+      out[key.slice(prefix.length)] = lines
+    }
+  }
+  return out
+}
+
+/** The first error key of a field in the server's order: itself, else its first row path. */
+export function firstErrorKey(errors: Record<string, string[]>, name: string): string | undefined {
+  if (errors[name]) {
+    return name
+  }
+  return Object.keys(errors).find((key) => key.startsWith(`${name}.`))
+}
+
+const NAME = /^[A-Za-z0-9_-]+$/
+
+/**
+ * Focuses the control of an error key. A row path (`content.3.answer`)
+ * focuses the nested control of that repeater row, found through the row's
+ * data attributes and the nested label's `for`; a plain name is focusField.
+ */
+export function focusError(prefix: string, key: string): boolean {
+  const parts = key.split('.')
+  if (parts.length < 3) {
+    return focusField(prefix, parts[0] ?? key)
+  }
+  const [field, index, nested] = parts as [string, string, string]
+  if (!NAME.test(field) || !/^\d+$/.test(index) || !NAME.test(nested)) {
+    return focusField(prefix, field)
+  }
+  const cell = document.querySelector(`[data-repeater="${field}"] [data-repeater-row="${index}"] [data-field="${nested}"]`)
+  const target = cell?.querySelector('label[for]')?.getAttribute('for')
+  const control =
+    (target ? document.getElementById(target) : null) ??
+    cell?.querySelector<HTMLElement>('input, select, textarea, [contenteditable="true"], button')
+  if (!control) {
+    return focusField(prefix, field)
+  }
+  if (!control.matches('input, select, textarea, button, [tabindex], [contenteditable="true"]')) {
+    control.setAttribute('tabindex', '-1')
+  }
+  control.focus()
+  return true
+}