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

modules/hosts/nixos/hetznix02/hardware-configuration.nix

@@ -4,34 +4,43 @@
 { lib, modulesPath, ... }:
 
 {
-  imports =
-    [ (modulesPath + "/profiles/qemu-guest.nix")
-    ];
+  imports = [
+    (modulesPath + "/profiles/qemu-guest.nix")
+  ];
 
   boot = {
     initrd = {
-      availableKernelModules = [ "xhci_pci" "virtio_scsi" "sr_mod" ];
+      availableKernelModules = [
+        "xhci_pci"
+        "virtio_scsi"
+        "sr_mod"
+      ];
       kernelModules = [ ];
     };
     kernelModules = [ ];
     extraModulePackages = [ ];
   };
 
-  fileSystems."/" =
-    { device = "/dev/disk/by-partlabel/disk-primary-root";
+  fileSystems = {
+    "/" = {
+      device = "/dev/disk/by-partlabel/disk-primary-root";
       fsType = "ext4";
     };
 
-  fileSystems."/boot" =
-    { device = "/dev/disk/by-partlabel/disk-primary-ESP";
+    "/boot" = {
+      device = "/dev/disk/by-partlabel/disk-primary-ESP";
       fsType = "vfat";
-      options = [ "fmask=0022" "dmask=0022" ];
+      options = [
+        "fmask=0022"
+        "dmask=0022"
+      ];
     };
 
-  fileSystems."/nix" =
-    { device = "/dev/disk/by-partlabel/disk-volume1-nix";
+    "/nix" = {
+      device = "/dev/disk/by-partlabel/disk-volume1-nix";
       fsType = "ext4";
     };
+  };
 
   swapDevices = [ ];