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/widgets.list-schema.json

@@ -0,0 +1,22 @@
+{
+  "data": {
+    "title": "Widgets",
+    "recordsPerPage": 20,
+    "perPageOptions": [],
+    "showSearch": true,
+    "showSetup": false,
+    "showCheckboxes": true,
+    "showSorting": true,
+    "searchTerm": "",
+    "toolbarButtons": ["create"],
+    "columns": [
+      { "key": "name", "label": "Name", "searchable": true, "sortable": true },
+      { "key": "code", "label": "Code", "searchable": true, "sortable": true },
+      { "key": "tags", "label": "Tags", "searchable": false, "sortable": false, "type": "relation", "relation": "tags", "select": "name" }
+    ],
+    "filters": [],
+    "rowActions": [],
+    "bulkActions": []
+  },
+  "meta": { "locale": "en" }
+}