fix(kan,puter): use .env + source for build env vars, fix Puter unreachable via IP

- kan: write .env before build, source it + export for build-time validation
  instead of inline exports (cleaner, consistent with codebase pattern)
- puter: set domain to container IP + experimental_no_subdomain so Puter
  accepts Host header when accessed via raw IP (default puter.localhost
  rejects all non-matching hosts)

install/puter-install.sh

@@ -40,7 +40,9 @@ msg_info "Configuring Application"
 cat <<EOF >/etc/puter/config.json
 {
   "config_name": "proxmox",
-  "allow_nipio_domains": true
+  "domain": "${LOCAL_IP}",
+  "http_port": 4100,
+  "experimental_no_subdomain": true
 }
 EOF
 msg_ok "Configured Application"