Extract ZEN_MODE handler to separate module

[mrolli]

Is your feature request related to a current shortcoming? Please describe.
The user-var-changed event handler for ZEN_MODE (lines 174-195 in wezterm.lua) is a self-contained feature that could be better organized as a separate plugin.

Describe the solution you'd like
Create a plugins/zenmode.lua module that:

• Encapsulates the ZEN_MODE logic
• Exports a setup function to register the event handler
• Keeps the main config file cleaner and more focused

Additional context
This follows the same pattern we established with trackinfo and weather plugins, making the codebase more modular and maintainable. The ZEN_MODE feature is standalone and doesn't need to be in the main config file.