build(deps): pin the stable overlay and stateVersion to 26.05

26.05 is the latest stable NixOS release as of now (26.11 is still the
in-development branch that nixos-unstable reports); the previous 25.05 pin
was a release behind.

- flake.nix / flake.lock: stable overlay tracks nixos-26.05.
- neogaia system.stateVersion and the base home.stateVersion set to 26.05
  (fresh install, so aligning to the current stable release).
- Refresh the "latest stable release" references in the project docs to
  match.

Other inputs are rolling branches (nixos-unstable, nixpkgs-unstable,
chaotic nyxpkgs-unstable) or master (home-manager, per spec), so they carry
no version to bump.

hosts/neogaia/default.nix

@@ -8,7 +8,7 @@
 {
   imports = [ ./hardware-configuration.nix ];
 
-  system.stateVersion = "25.05";
+  system.stateVersion = "26.05";
 
   boot.loader.systemd-boot.enable = true;
   boot.loader.efi.canTouchEfiVariables = true;