ui: Do not selected an index for the ToggleButtonGroup by default

[bnjjj]

ToggleButtonGroup: Changed default behavior to not pre-select any button index

• Changed selected_index field from usize to Option<usize>, defaulting to None instead of 0
• This prevents the first button (index 0) from being automatically selected when no explicit selection is made

Release Notes:

• Fixed: changed default behavior to ToggleButtonGroup to not pre-select any button index

[danilo-leal]

Hey @bnjjj, thanks for the PR! I'm curious, what is the scenario you're trying to fix where having an option selected by default isn't desirable?

[bnjjj]

@danilo-leal in this pr #42819 I have a button to filter by git remotes in branch picker

[danilo-leal]

Gotcha. Given that is just a filter that selects one option, we could use a regular Button for that, similar to the many toggle/filter buttons in the project search, for example. It makes sense to me to preserve the toggle button behavior where there's always a selected option by default... therefore, will close the PR due to that. (Also, we might change that branch picker design, too—we've been discussing internally about unifying the remote, branch, and worktree pickers into one). Thank you!