fix(theme): align nomarchy-theme-bg-next default with the schema

The script's jq fallback for .theme was hardcoded to "nord", which
drifted from the lib/state-schema.nix default of "summer-night". With
no state.json (or a blank one), nomarchy-theme-bg-next would look up
backgrounds under themes/palettes/nord/ while the rest of the system
treated summer-night as active — a quiet inconsistency that produced
"no backgrounds found" errors on a system that hadn't yet written a
theme to state.

Matched it to the schema default. lib/state-schema.nix remains the
single source of truth.

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

@@ -9,7 +9,7 @@ STATE_FILE="$STATE_DIR/state.json"
 mkdir -p "$STATE_DIR"
 [[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
 
-THEME_NAME=$(jq -r '.theme // "nord"' "$STATE_FILE")
+THEME_NAME=$(jq -r '.theme // "summer-night"' "$STATE_FILE")
 
 # Resolve themes directory (Built-in from Nix store via Home Manager, or user extra)
 if [ -d "$HOME/.config/nomarchy/themes/$THEME_NAME" ]; then