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/darwin/default.nix

@@ -1,8 +1,17 @@
-{ pkgs, hostname, username, ... }: {
+{
+  pkgs,
+  hostname,
+  username,
+  ...
+}:
+{
   system.primaryUser = username;
 
   environment = {
-    shells = with pkgs; [ bash zsh ];
+    shells = with pkgs; [
+      bash
+      zsh
+    ];
     pathsToLink = [
       "/Applications"
       "/share/zsh"
@@ -113,7 +122,10 @@
         "flox-cache-public-1:7F4OyH7ZCnFhcze3fJdfyXYLQw/aV7GEed86nQ7IsOs="
         "cache.thalheim.io-1:R7msbosLEZKrxk/lKxf9BTjOOH7Ax3H0Qj0/6wiHOgc="
       ];
-      trusted-users = [ "@admin" "${username}" ];
+      trusted-users = [
+        "@admin"
+        "${username}"
+      ];
     };
     extraOptions = ''
       # Generated by https://github.com/DeterminateSystems/nix-installer, version 0.11.0.