home: add nh + bump zen-browser

Enable the home-manager programs.nh module (installs nh, sets NH_FLAKE so
`nh home switch` finds this repo). Re-pin zen-browser to the 2026-06-28
revision to clear a stale twilight fixed-output-derivation hash mismatch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

home/common/programs/nix.nix

@@ -9,6 +9,13 @@
   programs.nix-index.enable = true;
   programs.nix-index-database.comma.enable = true;
 
+  # `nh` — friendlier wrapper around nix/home-manager (nh os/home switch).
+  # `flake` sets NH_FLAKE so `nh home switch` finds this repo without --flake.
+  programs.nh = {
+    enable = true;
+    flake = "/home/naps62/projects/nixos-config";
+  };
+
   home.packages = with pkgs; [
     nvd # version diff between generations (used by nh)
     nix-output-monitor # readable build output (nom; used by nh)