Add fish wrapper and make it my default shell

modules/system/common/default.nix

@@ -3,8 +3,11 @@
   configurationName,
   lib,
   pkgs,
+  inputs',
+  username,
   ...
 }: let
+  inherit (inputs'.settei.packages) fish;
   sharedConfig = {
     settei.user.config = {
       programs.git = {
@@ -22,7 +25,17 @@
           push.followTags = true;
         };
       };
+
+      programs.fish.enable = true;
     };
+
+    programs.fish.enable = true;
+    environment.shells = [fish];
+    users.users.${username}.shell = fish;
+
+    # NixOS' fish module doesn't allow setting what package to use for fish,
+    # so I need to override the fish package.
+    nixpkgs.overlays = [(_: _: {inherit fish;})];
   };
 
   linuxConfig = lib.optionalAttrs isLinux {