Allow overriding default browser via $BROWSER

[thejesus567]

Summary

When I first started using Omarchy, I had some issues trying to change the default browser.

Currently, the only documented way to do this is by running:

xdg-settings set default-web-browser

(as discussed here: #985)

This simple PR adds a more straightforward option: allowing users to set the $BROWSER environment variable in the default config file.

What changed

• omarchy-launch-browser now respects $BROWSER if it is set
• If $BROWSER is not set, it falls back to xdg-settings (current behavior)
• Added an example in the default config file to document this option

Why

Setting an environment variable is simpler, more explicit, and consistent with common Linux conventions. It also avoids having to run xdg-settings manually just to override the browser.

[dhh]

Appreciate the idea and the option, but I actually think this is more confusing for most users, as browsers already natively support the xdg-settings approach. They usually offer you "Make this your default browser" when you open them, and if you accept, they trigger that xdg-settings flow. So I prefer to stick to just one option here.