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.

flake.lock

@@ -94,16 +94,16 @@
     },
     "nixpkgs-stable": {
       "locked": {
-        "lastModified": 1767313136,
-        "narHash": "sha256-16KkgfdYqjaeRGBaYsNrhPRRENs0qzkQVUooNHtoy2w=",
+        "lastModified": 1784280462,
+        "narHash": "sha256-DtoqIqM7VkR6NxAkcLpMwmi02USwWb3JdmNGLyhthc0=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "ac62194c3917d5f474c1a844b6fd6da2db95077d",
+        "rev": "293d6abedf0478e681a4dfcfcb35b30fc796a32f",
         "type": "github"
       },
       "original": {
         "owner": "nixos",
-        "ref": "nixos-25.05",
+        "ref": "nixos-26.05",
         "repo": "nixpkgs",
         "type": "github"
       }