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/main.ts

@@ -2,6 +2,7 @@ import { createApp } from 'vue'
 import App from './App.vue'
 import { createAdminRouter } from './app/router'
 import { onUnauthorized } from './api/client'
+import { startBuildWatch } from './app/buildUpdate'
 import { loadFieldTypes } from './app/fieldTypes'
 import { loadStrings } from './app/i18n'
 import { expireSession, me } from './state/useAuth'
@@ -37,6 +38,9 @@ async function boot(): Promise<void> {
   app.use(router)
   await router.isReady()
   app.mount('#app')
+  // The new-version check; it stays off when the served index carries no
+  // build (the dev server).
+  startBuildWatch()
 }
 
 void boot()