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/views/SettingsIndexView.vue

@@ -0,0 +1,45 @@
+<script setup lang="ts">
+import { RouterLink } from 'vue-router'
+import { t } from '../app/i18n'
+import { iconFor } from '../app/icons'
+import { settingsGroups } from '../state/useSettings'
+
+// Settings index (D-21): every permitted settings page, grouped by category,
+// with its icon, label and description. The server filters the list.
+</script>
+
+<template>
+  <section class="flex flex-col gap-5">
+    <header>
+      <h1 class="text-[26px] font-bold tracking-[-0.02em]">{{ t('backend::lang.settings.title') }}</h1>
+    </header>
+    <p v-if="settingsGroups.length === 0" class="text-muted">{{ t('backend::lang.settings.empty') }}</p>
+    <section
+      v-for="group in settingsGroups"
+      :key="group.category"
+      data-settings-group
+      class="flex flex-col gap-3"
+    >
+      <h2 v-if="group.category" class="text-[13px] font-semibold tracking-[0.02em] text-muted uppercase">
+        {{ group.category }}
+      </h2>
+      <div class="grid grid-cols-1 gap-3 min-[900px]:grid-cols-2">
+        <RouterLink
+          v-for="entry in group.entries"
+          :key="entry.code"
+          :to="`/settings/${encodeURIComponent(entry.code)}`"
+          :data-settings="entry.code"
+          class="flex items-start gap-3.5 rounded-card border border-border bg-surface p-5 text-text no-underline shadow-card hover:bg-hover"
+        >
+          <span class="flex size-10 shrink-0 items-center justify-center rounded-inner bg-subtle text-muted">
+            <component :is="iconFor(entry.icon)" :size="20" aria-hidden="true" />
+          </span>
+          <span class="flex min-w-0 flex-col">
+            <span class="font-semibold">{{ entry.label }}</span>
+            <span v-if="entry.description" class="text-[13px] text-muted">{{ entry.description }}</span>
+          </span>
+        </RouterLink>
+      </div>
+    </section>
+  </section>
+</template>