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/composables/useToasts.ts

@@ -0,0 +1,23 @@
+export interface Toast {
+  id: number;
+  msg: string;
+  kind: "ok" | "bad";
+}
+
+export function useToasts() {
+  const toasts = useState<Toast[]>("app-toasts", () => []);
+
+  function push(msg: string, kind: "ok" | "bad" = "ok"): void {
+    const id = Date.now() + Math.floor(Math.random() * 1000);
+    toasts.value.push({ id, msg, kind });
+    setTimeout(() => {
+      toasts.value = toasts.value.filter((t) => t.id !== id);
+    }, 2600);
+  }
+
+  function dismiss(id: number): void {
+    toasts.value = toasts.value.filter((t) => t.id !== id);
+  }
+
+  return { toasts, push, dismiss };
+}