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/fish.nix

@@ -59,6 +59,10 @@
       if test -d /opt/homebrew/bin
         fish_add_path -g /opt/homebrew/bin /opt/homebrew/sbin
       end
+      
+      # Set default editor
+      set -gx EDITOR nvim
+      set -gx VISUAL nvim
     '';
   };