feat(installer): add one-file installer build and custom install ISO

Replace old_installer.py with a curses TUI installer packaged as a
one-file PyInstaller binary (python/installer/build.py, wrapped by
python/installer/package.nix). The default .#installer is patched to run
on foreign Linux live media; the new .#installer-nixos variant keeps its
Nix store interpreter so it runs on NixOS.

Add systems/iso, a minimal NixOS install CD with kernel 6.18 and ZFS 2.4
matching the deployed systems and the installer on PATH; build it with
nix build .#iso. Shared logging and subprocess helpers move out of
common.py into python/logging_config.py and python/process.py.

python/common.py

@@ -3,28 +3,20 @@
 from __future__ import annotations
 
 import logging
-import sys
 from datetime import UTC, datetime
 from os import getenv
 from subprocess import PIPE, Popen
 
 from apprise import Apprise
 
+from python.logging_config import configure_logger as _configure_logger
+
 logger = logging.getLogger(__name__)
 
 
 def configure_logger(level: str = "INFO") -> None:
-    """Configure the logger.
-
-    Args:
-        level (str, optional): The logging level. Defaults to "INFO".
-    """
-    logging.basicConfig(
-        level=level,
-        datefmt="%Y-%m-%dT%H:%M:%S%z",
-        format="%(asctime)s %(levelname)s %(filename)s:%(lineno)d - %(message)s",
-        handlers=[logging.StreamHandler(sys.stdout)],
-    )
+    """Configure the logger."""
+    _configure_logger(level)
 
 
 def bash_wrapper(command: str) -> tuple[str, int]: