ci(deploy): Nix flake + GitHub Actions pipeline for hermyhq (Next.js 16 standalone)

- flake.nix: build hermyhq via npm ci + next build (standalone), package .next/standalone + static + public behind /nix/var/nix/profiles/hermyhq
- next.config.ts: output: 'standalone' (Nix deploy bundles self-contained server)
- .github/workflows/deploy.yml: nix-installer (DeterminateSystems) + attic push pub:asepharyana + VPS nix-store realise (fallback ssh copy) + nix-env profile + systemctl restart hermyhq + GC cleanup

next.config.ts

@@ -2,7 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   // Vercel-compatible settings
-  output: undefined, // default — Vercel handles this automatically
+  output: "standalone", // Nix deploy bundles the standalone server
   images: {
     unoptimized: false,
   },