nix: factor runtime deps into runtime-deps.nix

[jpds]

Motivation

Extract the runtime dependency list from package.nix into a shared nix/runtime-deps.nix so shell.nix can include the same deps.

I noticed whilst testing the battery PR that my night light stopped running as wlsunset was no longer in the $PATH.

Type of Change

Mark the relevant option with an "x".

• Bug fix
• New feature
• Breaking change
• Refactoring

Related Issue

• Closes #(issue number) (if any)

Testing

Describe how you tested your changes and mark the relevant items.

• Tested on niri
• Tested on Hyprland
• Tested on sway
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors (if applicable)

Screenshots / Videos

If applicable, include screenshots or videos to help illustrate your changes.

Checklist

• Code follows project style guidelines
• Self-reviewed my code
• No new warnings or errors
• Documentation or comments updated (if relevant)