Set nvim as default editor across all configurations

- Add EDITOR and VISUAL environment variables to fish shell init
- Add system-level EDITOR and VISUAL variables for macOS and WSL
- Ensures nvim is used as default editor for git, system tools, and applications
- Maintains existing neovim defaultEditor configuration

nixos/hosts/macos.nix

@@ -46,6 +46,8 @@
   # Environment
   environment.variables = {
     DBT_USER = "DNTH";
+    EDITOR = "nvim";
+    VISUAL = "nvim";
   };
 
   environment.systemPackages = with pkgs; [