refactor: implement component-based architecture for enhanced maintainability

- Reorganize directory structure into core/, features/, and themes/
- Colocate application Nix logic, configs, scripts, and theme overrides
- Implement 'Inversion of Control' for theming: apps now pull theme-specific layouts
- Update flake.nix and shared library paths to match the new structure
- Document the new Feature-Centric architecture in README.md

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

@@ -0,0 +1,57 @@
+#!/usr/bin/env bash
+
+# Cycles through the background images available for the current theme.
+# Declarative + Hybrid (instant swww) for Nomarchy NixOS.
+
+STATE_DIR="$HOME/.config/nomarchy"
+STATE_FILE="$STATE_DIR/state.json"
+mkdir -p "$STATE_DIR"
+[[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
+
+THEME_NAME=$(jq -r '.theme // "nord"' "$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
+  THEMES_DIR="$HOME/.config/nomarchy/themes"
+else
+  THEMES_DIR="$HOME/.local/share/nomarchy/themes"
+fi
+
+BG_DIR="$THEMES_DIR/$THEME_NAME/backgrounds"
+
+if [ ! -d "$BG_DIR" ]; then
+    notify-send "No background directory found for theme $THEME_NAME"
+    exit 1
+fi
+
+mapfile -t BACKGROUNDS < <(ls "$BG_DIR" | sort)
+TOTAL=${#BACKGROUNDS[@]}
+
+if (( TOTAL == 0 )); then
+    notify-send "No backgrounds found in $BG_DIR"
+    exit 1
+fi
+
+CURRENT_BG=$(jq -r '.wallpaper' "$STATE_FILE")
+INDEX=-1
+for i in "${!BACKGROUNDS[@]}"; do
+    if [[ "$BG_DIR/${BACKGROUNDS[$i]}" == "$CURRENT_BG" ]]; then
+        INDEX=$i
+        break
+    fi
+done
+
+NEXT_INDEX=$(((INDEX + 1) % TOTAL))
+NEW_BG="$BG_DIR/${BACKGROUNDS[$NEXT_INDEX]}"
+
+TMP_JSON=$(mktemp)
+jq --arg wp "$NEW_BG" '.wallpaper = $wp' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
+
+# Instant feedback via swww
+if pgrep -x swww-daemon >/dev/null; then
+    swww img "$NEW_BG" --transition-type outer --transition-pos 0.85,0.97 --transition-step 90
+else
+    swww init && swww img "$NEW_BG"
+fi
+
+echo "Background set to $NEW_BG declaratively."