wolf: persist RetroArch shaders & overlays; back up bios + shaders/overlays

Mount retroarch/shaders and retroarch/overlays from the game drive into
~/.config/retroarch/{shaders,overlays} for both ES-DE and standalone
RetroArch (sibling to the existing cores mount). Anything pulled from the
Online Updater or set in the Quick Menu now persists across sessions and
survives a Wolf/OS reinstall, so a CRT shader or bezel set is configured
once. Dirs are pre-created on the game drive; storage summary and README
updated, including a fallback note to repoint Settings -> Directory if
RetroArch ever writes elsewhere.

gaming-backup: add opt-in snapshots for emulator BIOS (bios/) and the
RetroArch shaders/overlays dirs (cores excluded — re-downloadable). New
BACKUP_BIOS and BACKUP_RA_SHADERS flags wired through the prompts,
backup.conf, summary, and the worker script.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XV8mwKLFwUKt94dAKh3k79

extras/backup_gaming.sh

@@ -120,6 +120,9 @@ if [ -n "${MC_BASE_DIR:-}" ]; then
     done
 fi
 [ "${BACKUP_SAVES:-no}" = yes ]  && snap "emulator-saves" "$GAME_STORAGE_DIR/saves"
+[ "${BACKUP_BIOS:-no}"  = yes ]  && snap "emulator-bios"  "$GAME_STORAGE_DIR/bios"
+[ "${BACKUP_RA_SHADERS:-no}" = yes ] && snap "retroarch-shaders"  "$GAME_STORAGE_DIR/retroarch/shaders"
+[ "${BACKUP_RA_SHADERS:-no}" = yes ] && snap "retroarch-overlays" "$GAME_STORAGE_DIR/retroarch/overlays"
 [ "${BACKUP_STEAM:-no}" = yes ]  && snap "steam-userdata" "$GAME_STORAGE_DIR/steam"
 [ "${BACKUP_MEDIA:-no}" = yes ]  && snap "es-de-media"    "$GAME_STORAGE_DIR/media"
 [ "${BACKUP_WOLF:-no}"  = yes ]  && snap "wolf-state"     "$WOLF_STATE_DIR"