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/shell/PluginRail.vue

@@ -1,15 +1,20 @@
 <script setup lang="ts">
 import { computed } from 'vue'
 import { RouterLink, useRoute } from 'vue-router'
-import { Sun } from '@lucide/vue'
+import { Settings, Sun } from '@lucide/vue'
 import { t } from '../../app/i18n'
 import { iconFor } from '../../app/icons'
 import { activeEntry, firstControllerPath, railEntries } from '../../state/useNavigation'
+import { hasSettings } from '../../state/useSettings'
 
 const route = useRoute()
 
 const active = computed(() => activeEntry(String(route.params.vendor ?? ''), String(route.params.plugin ?? '')))
 
+// Ustawienia (D-21) is pinned to the bottom and shown only when the admin may
+// manage at least one settings page.
+const inSettings = computed(() => route.path === '/settings' || route.path.startsWith('/settings/'))
+
 const items = computed(() =>
   railEntries.value.map((entry) => ({
     entry,
@@ -39,5 +44,17 @@ const items = computed(() =>
       <component :is="item.icon" :size="20" aria-hidden="true" />
       <span>{{ item.entry.label }}</span>
     </RouterLink>
+    <span class="flex-1" aria-hidden="true" />
+    <RouterLink
+      v-if="hasSettings"
+      to="/settings"
+      data-rail-settings
+      :aria-current="inSettings ? 'page' : undefined"
+      :class="inSettings ? 'bg-accent-soft font-bold text-accent' : 'text-side-text hover:bg-side-hover hover:text-white'"
+      class="flex w-[68px] flex-col items-center gap-1 rounded-inner px-1.5 py-2 text-center text-[11px] leading-[1.2] tracking-[-0.01em] no-underline"
+    >
+      <Settings :size="20" aria-hidden="true" />
+      <span>{{ t('backend::lang.nav.settings') }}</span>
+    </RouterLink>
   </nav>
 </template>