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.lock

@@ -2,16 +2,16 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1699725108,
-        "narHash": "sha256-NTiPW4jRC+9puakU4Vi8WpFEirhp92kTOSThuZke+FA=",
+        "lastModified": 1781216227,
+        "narHash": "sha256-9mUW6gNwoN2SWc/l0fW4svPNOulXLl8ijqKyeSOGgJE=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "911ad1e67f458b6bcf0278fa85e33bb9924fed7e",
+        "rev": "a0374025a863d007d98e3297f6aa46cc3141c2f0",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixpkgs-unstable",
+        "ref": "nixos-26.05",
         "repo": "nixpkgs",
         "type": "github"
       }