fix(home-manager): back up clobbered dotfiles on activation

Set home-manager.backupFileExtension = "backup" on the NixOS and Darwin base modules. Existing machines carry hand-written ~/.zshrc and ~/.zshenv that home-manager would refuse to overwrite, aborting activation. With a backup extension, the originals are saved as .backup and activation proceeds. The oh-my-zsh setup is already declared in lyrathorpe/home/shell.nix, so the generated files supersede the old ones.

flake.nix

@@ -79,6 +79,9 @@
           {
             home-manager.useGlobalPkgs = true;
             home-manager.useUserPackages = true;
+            # Back up pre-existing dotfiles (e.g. .zshrc) instead of aborting
+            # activation when home-manager would overwrite them.
+            home-manager.backupFileExtension = "backup";
           }
         ];
 
@@ -117,6 +120,9 @@
           {
             home-manager.useGlobalPkgs = true;
             home-manager.useUserPackages = true;
+            # Back up pre-existing dotfiles (e.g. .zshrc) instead of aborting
+            # activation when home-manager would overwrite them.
+            home-manager.backupFileExtension = "backup";
           }
         ];