config: migrate markdown highlighting to Zola 0.22 schema

The nixpkgs bump to nixos-26.05 pulls in Zola 0.22, which replaced the
syntect highlighting backend with Giallo and changed the config schema.
Replace the removed `highlight_code` key with a `[markdown.highlighting]`
table using the `one-dark-pro` theme.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

flake.nix

@@ -1,5 +1,5 @@
 {
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
   inputs.theme_anemone = {
     url = "github:Speyll/anemone";
     flake = false;