feat(10-01): serve the embedded admin SPA at backend.uri with cookie login

- backend.uri prefix (default /backend) mounts the admin API at {prefix}/api/v1
  and the embedded SPA shell at {prefix} with an api/ JSON 404 fallback
- cookie transport: an X-Requested-With login sets the HttpOnly summer_admin
  cookie and returns no token; the backend guard reads the cookie after Bearer
- CSRF wrapper refuses cookie-only POST/PUT/DELETE without X-Requested-With
- boardwalk package embeds boardwalk/dist, rewrites index.html once per prefix
  and sets cache and security headers
- framework admin OpenAPI pipeline (swag, swagger2openapi, openapi-typescript)
  with prefix-relative paths and typed envelopes for the tracer routes
- admin/ Vite SPA: login, plugin rail, section panel and read-only list
  through the openapi-fetch client typed by the generated schema

admin/src/App.vue

@@ -0,0 +1,15 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { RouterView, useRoute } from 'vue-router'
+import AppShell from './components/shell/AppShell.vue'
+
+const route = useRoute()
+const inShell = computed(() => route.meta.shell === true)
+</script>
+
+<template>
+  <AppShell v-if="inShell">
+    <RouterView />
+  </AppShell>
+  <RouterView v-else />
+</template>