Convert tmux config to Nix and personalize system configuration

nixos/modules/shell.nix

@@ -6,7 +6,7 @@
     defaultShell = lib.mkOption {
       type = lib.types.str;
       default = "zsh";
-      description = "Default shell (bash, zsh, fish)";
+      description = "Default shell (dash, zsh, fish)";
     };
   };
 
@@ -15,13 +15,11 @@
   in lib.mkIf cfg.enable {
 
     programs.zsh.enable = cfg.defaultShell == "zsh";
-    programs.bash.enable = true;
+    programs.dash.enable = true;
     programs.fish.enable = cfg.defaultShell == "fish";
 
     # Common shell packages
     environment.systemPackages = with pkgs; [
-      starship
-      direnv
     ];
 
   };