Add config save/load to township fighters generator

CLI: -s/--save FILE writes selected generation options to a JSON config
and exits; -c/--config FILE loads them as defaults (command-line args
still override, via a two-phase parse).

Web UI: "Save config" button writes the current form options to a file
on the server (relative paths land in the output dir, absolute paths as-is)
via a new /save-config endpoint. Web- and CLI-saved configs are
interchangeable (shared CONFIG_FIELDS / canonical mapping).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>