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/tests/fixtures/navigation.json

@@ -0,0 +1,55 @@
+{
+  "data": [
+    {
+      "code": "demo",
+      "label": "Demo",
+      "icon": "disc-3",
+      "order": 100,
+      "controller": "acme.demo.widgets",
+      "sideMenu": [
+        {
+          "code": "widgets",
+          "label": "Widgets",
+          "icon": "tags",
+          "order": 0,
+          "controller": "acme.demo.widgets",
+          "sideMenu": []
+        },
+        {
+          "code": "gadgets",
+          "label": "Gadgets",
+          "icon": "icon-archive",
+          "order": 0,
+          "controller": "acme.demo.gadgets",
+          "sideMenu": []
+        }
+      ]
+    },
+    {
+      "code": "tools",
+      "label": "Tools",
+      "icon": "unknown-icon-name",
+      "order": 300,
+      "controller": "acme.tools.hammers",
+      "sideMenu": [
+        {
+          "code": "hammers",
+          "label": "Hammers",
+          "icon": "puzzle",
+          "order": 0,
+          "controller": "acme.tools.hammers",
+          "sideMenu": []
+        }
+      ]
+    },
+    {
+      "code": "hidden",
+      "label": "Hidden",
+      "icon": "users",
+      "order": 200,
+      "controller": "acme.hidden.items",
+      "sideMenu": []
+    }
+  ],
+  "meta": { "locale": "en" }
+}