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/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "smirk-admin-ui",
+  "private": true,
+  "version": "0.3.0",
+  "type": "module",
+  "description": "Operator console SPA, embedded into smirk-backend-core and served at /admin.",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build"
+  },
+  "dependencies": {
+    "preact": "^10.24.3"
+  },
+  "devDependencies": {
+    "typescript": "^5.6.3",
+    "vite": "^5.4.10"
+  }
+}