feat: add Nix flake for reproducible builds and deployment

- Add flake.nix with packages for all 5 apps (landing, gacha, backoffice, dimentorin, hackathon)
- Add NixOS modules for deployment (systemd service for Next.js, nginx for Vite apps)
- Add mkHackathonWithEnv function for build-time environment variables
- Add development shell with Node.js 22, npm, bun

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nix/mkViteApp.nix

@@ -0,0 +1,33 @@
+# Helper function to build Vite-based static apps
+{ pkgs, src, npmDepsHash }:
+
+{ name, buildScript, envVars ? {} }:
+
+pkgs.buildNpmPackage {
+  pname = "imphnen-${name}";
+  version = "0.0.1";
+  inherit src npmDepsHash;
+
+  nativeBuildInputs = with pkgs; [
+    nodejs_22
+    nodePackages.npm
+  ];
+
+  buildPhase = ''
+    runHook preBuild
+    export NX_DAEMON=false
+    export HOME=$TMPDIR
+    ${pkgs.lib.concatStringsSep "\n" (pkgs.lib.mapAttrsToList (k: v: "export ${k}=\"${v}\"") envVars)}
+    npm run ${buildScript}
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p $out
+    cp -r dist/apps/${name}/* $out/
+    runHook postInstall
+  '';
+
+  dontNpmBuild = true;
+}