Add install.sh and a Home Manager flake template

install.sh installs Nix (nix-installer, steam-deck planner, flakes
enabled) and activates a Home Manager configuration, creating one from
the new flake template if there is none; uninstall stops Home Manager's
user services, runs home-manager uninstall and removes Nix and per-user
leftovers; status shows the current state. Works piped from curl.

flake.nix

@@ -31,5 +31,11 @@
     homeManagerModules = modules // {
       default = { imports = builtins.attrValues modules; };
     };
+
+    # nix flake init -t github:lhns/steam-frame-nix
+    templates.default = {
+      path = ./template;
+      description = "Standalone Home Manager configuration for SteamOS using steam-frame-nix";
+    };
   };
 }