_find_dck_file only searched CFG.precon_dir / CFG.forge_decks_dir, both
of which are empty when the server is started without CLI args. This
caused all precon deck lookups to return 0 cards.
Fix: add _REPO_ROOT and _BUILTIN_DECK_DIRS constants, then append
precon-decks/, sample-decks/, imported-decks/ to the search list
whenever they exist on disk, regardless of whether CFG paths are set.