1. Security: Navidrome no longer exposed to network (localhost only)
2. Maintainability: Single username definition in flake.nix:45
3. Organization: Kernel params now in configuration.nix, session vars in
desktop.nix
4. Automation: Flathub repository added automatically on activation
5. Cleanup: Removed duplicate programs.dconf.enable
Consistently format module argument blocks and convert aligned
package comments to inline comments. Trim trailing whitespace and
minor layout tweaks across modules. Bump opencode rev, narHash and
lastModified in flake.lock and remove the original dev ref.
