Add linting, formatting, and CI with fixes for all warnings

Infrastructure:
- Add deadnix, nixfmt, and statix to flake inputs
- Add formatter output to flake for nix fmt support
- Add deadnix, nixfmt, statix to Home Manager packages
- Add GitHub Actions workflow for CI validation
- Add .pre-commit-config.yaml with hooks for nixfmt, deadnix, and statix
- Support x86_64-darwin in formatter

Statix fixes (W10/W20 warnings):
- Remove unused lambda argument from nixpkgs-settings.nix
- Merge repeated keys in hardware-configuration.nix files (boot.initrd, boot, fileSystems)
- Merge repeated keys in nixnuc/default.nix (services, virtualisation)
- Merge repeated keys in rainbow-planet/default.nix (desktopManager)
- Merge repeated keys in home/general/default.nix (home)

Deadnix fixes (unused declarations):
- Remove unused pkgs/lib/username/http_port arguments from various files
- Fix unused final parameter in overlay functions (final -> _final)

CI/pre-commit fixes:
- Fix pre-commit statix config: add pass_filenames: false
- Fix CI workflow: use nix run nixpkgs# prefix and --ci flag for nixfmt

lib/mkHomeConfig.nix

@@ -1,29 +1,39 @@
-{ inputs, ... }: {
-  mkHomeConfig = {
-    homeDirectory,
-    system,
-    username,
-  }: inputs.home-manager.lib.homeManagerConfiguration {
-    extraSpecialArgs = { inherit inputs homeDirectory system username; };
+{ inputs, ... }:
+{
+  mkHomeConfig =
+    {
+      homeDirectory,
+      system,
+      username,
+    }:
+    inputs.home-manager.lib.homeManagerConfiguration {
+      extraSpecialArgs = {
+        inherit
+          inputs
+          homeDirectory
+          system
+          username
+          ;
+      };
 
-    pkgs = inputs.nixpkgs.legacyPackages.${system};
+      pkgs = inputs.nixpkgs.legacyPackages.${system};
 
-    # Specify your home configuration modules here, for example,
-    # the path to your home.nix.
-    modules = [
-      ./nixpkgs-settings.nix
-      ../modules/shared/home/general
-      ../modules/hosts/home-manager-only
-      ../modules/hosts/home-manager-only/home-${username}.nix
+      # Specify your home configuration modules here, for example,
+      # the path to your home.nix.
+      modules = [
+        ./nixpkgs-settings.nix
+        ../modules/shared/home/general
+        ../modules/hosts/home-manager-only
+        ../modules/hosts/home-manager-only/home-${username}.nix
 
-      {
-        home = {
-          username = "${username}";
-          homeDirectory = "${homeDirectory}";
-        };
-      }
+        {
+          home = {
+            username = "${username}";
+            homeDirectory = "${homeDirectory}";
+          };
+        }
 
-      inputs.sops-nix.homeManagerModules.sops
-    ];
-  };
-}
+        inputs.sops-nix.homeManagerModules.sops
+      ];
+    };
+}