feat(upstream): switch to remote git repository for engine

- Update installer to use git+https://git.bemagri.xyz/bernardo/Nomarchy.git
- Remove redundant bundling/copying of the engine to /etc/nixos/nomarchy
- Expose all themes via xdg.dataFile for script accessibility
- Update theme scripts to resolve directories via local share instead of hardcoded system paths
- Update documentation to reflect the new remote-first architecture

bin/nomarchy-theme-bg-next

@@ -10,12 +10,11 @@ mkdir -p "$STATE_DIR"
 
 THEME_NAME=$(jq -r '.theme // "nord"' "$STATE_FILE")
 
-if [ -d "/etc/nixos/nomarchy/themes" ]; then
-  THEMES_DIR="/etc/nixos/nomarchy/themes"
-elif [ -d "/etc/nomarchy/themes" ]; then
-  THEMES_DIR="/etc/nomarchy/themes"
+# 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="$HOME/.config/nomarchy/themes"
 else
-  THEMES_DIR="/etc/nixos/themes"
+  THEMES_DIR="$HOME/.local/share/nomarchy/themes"
 fi
 
 BG_DIR="$THEMES_DIR/$THEME_NAME/backgrounds"