feat(10-03): tabs, toggles, relation fields, form lifecycle and settings pages

- FormTabs groups fields by tab (untabbed fields in the default tab) as a
  segmented tablist; a tab holding invalid fields after a 422 shows a
  count badge and the form switches to the first invalid field
- switch, checkbox and relation join the renderer registry: toggle cards
  (role=switch, 20px checkbox) keep a numeric value numeric; relation
  fields are read-only labels from meta.labels, a searchable single
  select over fields/{field}/options with emptyOption first, or ordered
  removable chips with an appending search (300 ms debounce, 20 per
  page, more on scroll or the more action)
- FormView gets the back button, record title and update subtitle, a
  sticky footer (Usuń with deleteConfirm then DELETE, Anuluj, Zapisz i
  zamknij to the mapped redirectClose, Zapisz), and asks before leaving a
  dirty form on any route change plus a beforeunload guard
- Settings: the rail pins Ustawienia to the bottom when /settings is
  non-empty; /settings lists pages by category; /settings/:code renders
  the settings schema through FormGrid and the registry, PUTs the values
  and maps a 422 like the record form
- New backend::lang keys (form.more_options, tab_default, discard,
  settings.back); form and settings smoke tests; boardwalk/dist rebuilt

admin/src/components/form/formState.ts

@@ -5,6 +5,30 @@ import { isRegistered } from './registry'
 
 export type FormMode = 'create' | 'update'
 
+/** One form tab: the YAML tab label, or the default tab for untabbed fields. */
+export interface TabItem {
+  key: string
+  label: string
+  errors: number
+}
+
+/** Key of the tab that holds fields without a `tab`. */
+export const DEFAULT_TAB = 'default'
+
+/** Tab key of a field: its tab label, or the default tab. */
+export function tabOf(field: FormField): string {
+  return field.tab ? `tab:${field.tab}` : DEFAULT_TAB
+}
+
+/** DOM ids of the tab button and its panel, by tab position. */
+export function tabDomId(prefix: string, index: number): string {
+  return `${prefix}-tab-${index}`
+}
+
+export function panelDomId(prefix: string, index: number): string {
+  return `${prefix}-panel-${index}`
+}
+
 /** Winter `context:` semantics: no context shows the field everywhere. */
 export function contextAllows(field: FormField, mode: FormMode): boolean {
   const context = field.context