Add shared Nix configuration for machine setups

Introduces a shared configuration file to streamline machine-specific settings for NixOS deployments. This includes:

- Hostname configuration
- Common system packages (vim, git, htop)
- SSH service enablement
- Nginx setup with virtual host configuration
- Firewall rules for HTTP/HTTPS access

Updates machine-specific configurations to import shared settings, reducing redundancy and improving maintainability.

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

config/shared.nix

@@ -0,0 +1,33 @@
+{ config, pkgs, hostName, ... }:
+
+{
+  # Set hostname (passed as parameter)
+  networking.hostName = hostName;
+
+  # System packages
+  environment.systemPackages = with pkgs; [
+    vim
+    git
+    htop
+  ];
+
+  # Enable SSH
+  services.openssh.enable = true;
+
+  # Enable and configure nginx
+  services.nginx = {
+    enable = true;
+    virtualHosts."${hostName}" = {
+      root = "/var/src/web-app-dist";
+      locations."/" = {
+        index = "index.html";
+      };
+    };
+  };
+
+  # Open firewall for HTTP/HTTPS
+  networking.firewall.allowedTCPPorts = [ 80 443 ];
+
+  # NixOS release version
+  system.stateVersion = "25.05";
+}