feat(settings): add on_page_change callback to Settings widget

[ralphschindler]

Description

Settings stores its selected page in SettingsState, which is pub(super) and cannot be read by consumers. The only external control point is default_selected_index, which only applies on first initialization.

This makes it impossible to implement patterns like restoring the last-visited settings page when a user navigates away and returns — a common need for any app that lazily shows/hides the settings view.

This PR adds an optional on_page_change callback that fires whenever the user selects a top-level page in the sidebar:

Note: I opted to test this in the story since it did not appear unit style tests were favored in the repo, specifically this settings/ directory. I can add a test (back) in settings/ if that is preferable.

Settings::new("my-settings")
    .default_selected_index(SelectIndex { page_ix: last_page, group_ix: None })
    .on_page_change(move |page_ix, cx| {
        // store page_ix externally; pass it back via default_selected_index on next render
    })

Screenshot

not applicable.

Break Changes

none.

How to Test

1. Create a Settings widget with .on_page_change(|ix, _cx| println!("page: {ix}"))
2. Click between pages in the sidebar — callback should fire with the correct zero-based index
3. Confirm existing Settings usage without .on_page_change compiles and behaves identically

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)