feat(boardwalk): expose the admin build id and show a reload banner when a new build is deployed (quick-261028-rqb)

- boardwalk.BuildID hashes the dist tree plus the binary's build info once
  per handler; X-Summer-Build on every response, {prefix}/build.json and the
  summer-admin-build meta carry it
- the SPA reads the booted build, re-checks on focus, visibility and every
  5 minutes, and shows UpdateBanner on a different build

admin/src/components/ui/UpdateBanner.vue

@@ -0,0 +1,41 @@
+<script setup lang="ts">
+import { X } from '@lucide/vue'
+import { dismissNewBuild, newBuildAvailable } from '../../app/buildUpdate'
+import { t } from '../../app/i18n'
+import { page } from '../../state/usePreferences'
+
+// New-version banner (buildUpdate.ts): shown while the server answers a build
+// different from the one this tab booted with. It never reloads by itself;
+// Reload goes through page.reload, so a dirty form's beforeunload prompt
+// still asks, and Close hides it until another build is deployed.
+</script>
+
+<template>
+  <!-- Bottom right, 88px up so it clears the sticky form footer, like the toasts. -->
+  <div
+    v-if="newBuildAvailable"
+    data-update-banner
+    role="status"
+    aria-live="polite"
+    class="fixed right-4 bottom-[88px] z-50 flex max-w-[420px] items-center gap-3 rounded-[14px] bg-[#1d2740] py-3 pr-2.5 pl-3.5 text-white shadow-toast"
+  >
+    <span class="font-medium">{{ t('backend::lang.version.available') }}</span>
+    <button
+      type="button"
+      data-update-reload
+      class="shrink-0 rounded-pager bg-accent px-3 py-1.5 font-medium text-on-accent transition-opacity duration-150 ease-out hover:opacity-90"
+      @click="page.reload()"
+    >
+      {{ t('backend::lang.version.reload') }}
+    </button>
+    <button
+      type="button"
+      data-update-close
+      :aria-label="t('backend::lang.form.close')"
+      class="flex size-8 shrink-0 items-center justify-center rounded-pager text-[#c3cbda] transition-colors duration-150 ease-out hover:bg-side-hover hover:text-white"
+      @click="dismissNewBuild()"
+    >
+      <X :size="16" aria-hidden="true" />
+    </button>
+  </div>
+</template>