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

go.mod

@@ -2,6 +2,9 @@ module git.golem15.com/golem15/summercms
 
 go 1.27.0
 
+// The admin SPA's npm tree may ship .go files; keep ./... out of it.
+ignore ./admin/node_modules
+
 require (
 	github.com/disintegration/imaging v1.6.2
 	github.com/fsnotify/fsnotify v1.10.1