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>

.gitignore

@@ -19,3 +19,4 @@ Thumbs.db
 
 # Local user config (generated by installer)
 config.local.nix
+test-vm/