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/commands_test.go

@@ -107,7 +107,7 @@ func TestAdminResetPasswordCommand(t *testing.T) {
 		t.Fatal(err)
 	}
 	guard := bouncer.NewBackendJWTGuard(adminTestSecret, cabana.BackendUsers{DB: gdb}, nil, nil)
-	req := httptest.NewRequest(http.MethodGet, "/_admin/api/v1/auth/me", nil)
+	req := httptest.NewRequest(http.MethodGet, adminAPI("/auth/me"), nil)
 	req.Header.Set("Authorization", "Bearer "+token)
 	if _, err := guard.Authenticate(req); err != nil {
 		t.Fatalf("token before reset: %v", err)