Show the API reference with Redoc

Replace the hand-built reference page with Redoc rendering the generated
OpenAPI document. The bundle is fetched by package.nix (pinned hash) and
served from the UI's own address; only that page gets a style nonce and
worker-src blob: in its CSP.

The spec now explains authentication, saved-not-applied, ETags (If-Match,
409) and errors, documents the ETag header, gives each operation a curl
sample, and no longer names MCP tools. Area GETs no longer emit
"parameters": null, which Redoc could not render.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/static/apiref.css

@@ -0,0 +1 @@
+body { margin: 0; }

pkgs/router-webui/static/apiref.js

@@ -0,0 +1,11 @@
+// Shows the OpenAPI document with Redoc (redoc.standalone.js).
+(function () {
+  var el = document.getElementById("redoc");
+  Redoc.init(el.dataset.spec, {
+    nonce: el.dataset.nonce,
+    hideDownloadButton: false,
+    expandResponses: "200",
+    pathInMiddlePanel: true,
+    sortPropsAlphabetically: false,
+  }, el);
+})();