Errors from loading and saving settings and themes. Typed at this seam so
callers can match on the failure; the binary’s eyre top level (CLI, main)
wraps it automatically via ?.
Path to kimün’s config directory (~/.config/kimun, or kimun_debug in
debug builds), creating it if needed. Single source of truth for the
debug/release directory name — used by the update module for the install
marker and update-state file.