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-set

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+
+# Set the system theme declaratively.
+# Usage: nomarchy-theme-set <theme-name>
+
+THEME_NAME="$1"
+
+if [[ -z $THEME_NAME ]]; then
+  echo "Usage: nomarchy-theme-set <theme-name>"
+  exit 1
+fi
+
+STATE_DIR="$HOME/.config/nomarchy"
+STATE_FILE="$STATE_DIR/state.json"
+
+# Resolve themes directory (Built-in from Nix store via Home Manager, or user extra)
+if [ -d "$HOME/.config/nomarchy/themes/$THEME_NAME" ]; then
+  THEMES_DIR="$HOME/.config/nomarchy/themes"
+else
+  THEMES_DIR="$HOME/.local/share/nomarchy/themes"
+fi
+
+mkdir -p "$STATE_DIR"
+[[ ! -f $STATE_FILE ]] && echo "{}" > "$STATE_FILE"
+
+if [ ! -d "$THEMES_DIR/$THEME_NAME" ] && ! [[ "$THEME_NAME" == "nord" ]]; then
+  echo "Theme '$THEME_NAME' not found in $THEMES_DIR"
+fi
+
+TMP_JSON=$(mktemp)
+jq --arg theme "$THEME_NAME" '.theme = $theme' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
+
+# Sync to system state if we have permissions (for system-level theming like browser policies)
+SYSTEM_STATE_FILE="/etc/nixos/state.json"
+if [ -w "$SYSTEM_STATE_FILE" ] || [ -w "/etc/nixos" ]; then
+    sudo jq --arg theme "$THEME_NAME" '.theme = $theme' "$SYSTEM_STATE_FILE" > /tmp/system-state.json 2>/dev/null && sudo mv /tmp/system-state.json "$SYSTEM_STATE_FILE" 2>/dev/null || true
+fi
+
+# Try to find a background for this theme
+BG_DIR="$THEMES_DIR/$THEME_NAME/backgrounds"
+if [ -d "$BG_DIR" ]; then
+  BG=$(ls "$BG_DIR" | head -n 1)
+  if [ -n "$BG" ]; then
+    TMP_JSON=$(mktemp)
+    jq --arg wp "$BG_DIR/$BG" '.wallpaper = $wp' "$STATE_FILE" > "$TMP_JSON" && mv "$TMP_JSON" "$STATE_FILE"
+  fi
+fi
+
+echo "Theme set to $THEME_NAME. Applying changes with env-update..."
+rm -rf "$HOME/.config/nomarchy/current/theme"
+env-update
+
+nomarchy-theme-set-templates
+
+nomarchy-hook theme-set "$THEME_NAME"