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

cabana/bulk_test.go

@@ -18,7 +18,7 @@ import (
 func TestBulkDeleteEmpty(t *testing.T) {
 	cap := &captureRouter{}
 	(&service{}).mount(cap)
-	key := "POST /_admin/api/v1/{vendor}/{plugin}/{controller}/bulk-delete"
+	key := "POST " + adminAPI("/{vendor}/{plugin}/{controller}/bulk-delete")
 	mw, ok := cap.middleware[key]
 	if !ok || !containsString(mw, "backend") {
 		t.Fatalf("missing %s in %v", key, cap.routes)