Add fresh install script for ISO installer

- Add installer/fresh-install.sh for installing from live ISO
- Handles disk partitioning, LUKS encryption, filesystem setup
- Generates minimal NixOS config for first boot
- Enable flakes in ISO environment
- Pass installer script content to ISO build via specialArgs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -105,7 +105,10 @@
       # ISO installer configuration
       installer = nixpkgs.lib.nixosSystem {
         system = "x86_64-linux";
-        specialArgs = {inherit inputs;};
+        specialArgs = {
+          inherit inputs;
+          installerScriptContent = builtins.readFile ./installer/fresh-install.sh;
+        };
         modules = [
           ./iso/default.nix
         ];