Add assertion that zsh is installed if we set it as our default shell

hosts/vm/default.nix

@@ -23,7 +23,7 @@
   modules.displayManager.displayManager = "gdm";
 
   modules.zsh.enable = true;
-  modules.shell.defaultShell = "bash";
+  modules.shell.defaultShell = "zsh";
 
   modules.pipewire.enable = true;
 }

modules/options/shell.nix

@@ -6,6 +6,7 @@
 with lib;
 let
   cfg = config.modules.shell;
+  zshCfg = config.modules.zsh;
 in
 {
   options.modules.shell = {
@@ -20,4 +21,12 @@ in
       '';
     };
   };
+
+  config.assertions = []
+    ++ (if cfg.defaultShell == "zsh"
+          then [{
+            assertion = zshCfg.enable;
+            message = "You have selected zsh as the default shell, but you haven't enabled it in your configuration.\n\nPlease enable it by adding `modules.zsh.enable = true`.";
+          }]
+        else []);
 }