feat: implement hybrid declarative state with automatic Nix sync

themes/engine/scripts/nomarchy-theme-bg-set

@@ -28,8 +28,7 @@ mkdir -p "$STATE_DIR"
 [[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
 
 # Persist the choice for the next rebuild.
-TMP_JSON=$(mktemp)
-jq --arg wp "$BACKGROUND" '.wallpaper = $wp' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
+nomarchy-state-write wallpaper "$BACKGROUND"
 
 # Create symlink to the new background
 ln -nsf "$BACKGROUND" "$CURRENT_BACKGROUND_LINK"