feat(admin): serve Operator Console SPA at /admin (rust-embed)

Preact + Vite standalone admin-ui bundled into the server binary via
rust-embed. Login is NIP-07 (Smirk wallet signs the NIP-98 admin
challenge); tabs cover instance status, admin keys, invite codes, and
the effective-config editor (live vs restart-required per field).

admin-ui/dist is committed because rust-embed reads it at compile time;
node_modules is gitignored (npm ci reproduces it). A lib test asserts
index.html and the /admin/ asset base are embedded so an unbuilt
console fails the build instead of an operator.

The console is served on the admin plane only (loopback bind), behind
the existing NIP-98 admin_guard + MAC allowlist.

admin-ui/dist/index.html

@@ -0,0 +1,14 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <meta name="robots" content="noindex" />
+    <title>Smirk Operator Console</title>
+    <script type="module" crossorigin src="/admin/assets/index-DLOlKNyc.js"></script>
+    <link rel="stylesheet" crossorigin href="/admin/assets/index-uKG4VNAS.css">
+  </head>
+  <body>
+    <div id="app"></div>
+  </body>
+</html>