Fix keyboard access for sidebar toggle

[lawrence3699]

Fixes #2615.

The sidebar toggle was rendered as a label for the hidden checkbox. Unlike the neighboring theme and search controls, it could not receive focus as a button, and pressing Enter did not toggle the sidebar.

This keeps the checkbox/label path for no-JavaScript output, but renders the JavaScript-enabled control as a native button. The button toggles the same checkbox and dispatches its existing change handler, so the current sidebar state, ARIA updates, and localStorage behavior stay on the existing path.

The GUI regression focuses the toggle directly because Tab traversal was not reliable in the headless test harness; it verifies the control can receive focus and that Enter toggles the sidebar closed and open.

Validation:

• cargo test --test gui -- sidebar.goml
• cargo test --test gui -- sidebar-nojs.goml
• npm run lint
• git diff --check

[ShamrockLee]

I'm going to test it this week. Thank you for making this documentation generator more accessible!

[Omnikron13]

If the fix doesn't touch the no-js version, should the toggle be un-hidden there so accessibility doesn't degrade without JS?