feat(admin): GitHub OAuth login, admin whitelist and Nuxt WebUI

Add an admin WebUI (Nuxt static SPA in admin/, served via the ASSETS
binding) for managing routes and viewing send logs. Access is gated by
GitHub OAuth plus an ADMIN_USER_IDS whitelist with cookie sessions
(admin-session.ts). Expose routes/logs CRUD API (admin-routes.ts), add a
discord-link mapping in token-store.ts, and mount admin routes with an
SPA assets fallback in the server.

admin/nuxt.config.ts

@@ -0,0 +1,22 @@
+export default defineNuxtConfig({
+  ssr: false,
+  compatibilityDate: "2025-01-01",
+  app: {
+    head: {
+      title: "WebHooker · Config Console",
+      link: [
+        {
+          rel: "stylesheet",
+          href: "https://fonts.googleapis.com/css2?family=Plus+Jakarta+Sans:wght@400;500;600;700;800&family=JetBrains+Mono:wght@400;500;600&display=swap",
+        },
+      ],
+    },
+  },
+  css: ["~/assets/css/main.css"],
+  devtools: { enabled: false },
+  runtimeConfig: {
+    public: {
+      apiBase: "/admin/api",
+    },
+  },
+});