Update flake.lock and force wallpaper style

Bump locked revisions for firefox‑addons, home‑manager, nixpkgs and
stylix in flake.lock, updating their lastModified timestamps, narHash
values and rev identifiers.

Change wallpaperStyle in boot.nix from lib.mkDefault to lib.mkForce to
ensure the centered style is always applied.

modules/core/boot.nix

@@ -19,7 +19,7 @@
           wallpapers = [
             ./../../wallpapers/attack-on-titan-mikasa-ackerman.jpg
           ];
-          wallpaperStyle = lib.mkDefault "centered";
+          wallpaperStyle = lib.mkForce "centered";
           backdrop = "${config.stylix.base16Scheme.base00}";
           interface = {
             branding = "GarandOS Bootloader";