Add new server modules and configurations for various services

- Introduced `default-creds` module for managing default credentials with Umami integration.
- Added `fail2ban` module for brute-force attack protection with configurable ban times.
- Created `firewall` module to enable and configure basic firewall settings.
- Implemented `gitea` module for self-hosted Git service with PostgreSQL backend.
- Developed `glance` module for a customizable dashboard with various widgets and themes.
- Added `iknowyou` module for a self-hosted password manager with production and demo environments.
- Introduced `kernel-hardening` module for enhancing kernel security settings.
- Created `mazanoke` module for a simple web application with Nginx integration.
- Added `mealie` module for a self-hosted meal planning application.
- Implemented `stirling-pdf` module for PDF generation service.
- Developed `umami` module for self-hosted analytics with secret management.
- Added `ssh` module for secure SSH configuration with user restrictions.
- Introduced `nixy` theme for a customized aesthetic experience across services.

nixos/nix.nix

@@ -0,0 +1,63 @@
+# Nix configuration for NixOS
+{
+  config,
+  inputs,
+  ...
+}: let
+  autoGarbageCollector = config.var.autoGarbageCollector;
+in {
+  # Ask for password once per SSH session (tied to the tty, expires when session closes)
+  security.sudo.extraConfig = ''
+    Defaults timestamp_type=tty,timestamp_timeout=-1
+  '';
+
+  security.sudo.extraRules = [
+    {
+      users = [config.var.username];
+      commands = [
+        {
+          command = "/run/current-system/sw/bin/nixos-rebuild";
+          options = ["NOPASSWD"];
+        }
+      ];
+    }
+  ];
+  nixpkgs.config = {
+    allowUnfree = true;
+    allowBroken = false;
+  };
+  nix = {
+    nixPath = ["nixpkgs=${inputs.nixpkgs}"];
+    channel.enable = false;
+    extraOptions = ''
+      warn-dirty = false
+    '';
+    settings = {
+      download-buffer-size = 262144000; # 250 MB (250 * 1024 * 1024)
+      auto-optimise-store = true;
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+      substituters = [
+        # high priority since it's almost always used
+        "https://cache.nixos.org?priority=10"
+
+        "https://hyprland.cachix.org"
+        "https://nix-community.cachix.org"
+        "https://numtide.cachix.org"
+      ];
+      trusted-public-keys = [
+        "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+        "numtide.cachix.org-1:2ps1kLBUWjxIneOy1Ik6cQjb41X0iXVXeHigGmycPPE="
+      ];
+    };
+    gc = {
+      automatic = autoGarbageCollector;
+      persistent = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+  };
+}