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/App.vue

@@ -3,6 +3,7 @@ import { computed, watchEffect } from 'vue'
 import { RouterView, useRoute } from 'vue-router'
 import { documentTitleForRoute } from './app/pageTitle'
 import AppShell from './components/shell/AppShell.vue'
+import UpdateBanner from './components/ui/UpdateBanner.vue'
 import { remountRevision } from './state/useRemount'
 
 const route = useRoute()
@@ -17,11 +18,13 @@ watchEffect(() => {
   <!-- Keyed by path, not full path: a new controller or record remounts its
        view, while list query changes (search, sort, page) keep it. The
        remount revision is part of the key, so a view can remount itself in
-       place (Create and new). -->
+       place (Create and new). The new-version banner sits outside the
+       v-if, so it also shows on the login screen. -->
   <AppShell v-if="inShell">
     <RouterView v-slot="{ Component, route: current }">
       <component :is="Component" :key="`${current.path}#${remountRevision}`" />
     </RouterView>
   </AppShell>
   <RouterView v-else />
+  <UpdateBanner />
 </template>