Add ARIA disclosure attributes to bottom-right panel tab buttons

[Yayabtw]

What

Adds aria-expanded and aria-controls attributes to the two tab buttons that toggle the live docs and process status panels in the bottom-right helpbox (BottomRightPanel.js).

Why

As described in #3274, these <button> elements act as toggle controls (open/close a panel) but expose no semantics to assistive technologies — screen-reader users cannot tell that the buttons control a different region of the page or whether the panel is currently expanded.

How

The interaction matches the W3C WAI-ARIA disclosure pattern:

• aria-expanded is bound to the open state (open_tab === "docs" / open_tab === "process"), so it flips between true and false as the user toggles the panel.
• aria-controls points to the <pluto-helpbox> element, which holds the disclosed content (the helpbox is the single container that swaps between docs view and status view depending on which tab is active). An id="pluto-helpbox" is added so the reference is resolvable.

Fix #3274

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/Yayabtw/Pluto.jl", rev="aria-attributes-tab-buttons")
julia> using Pluto

[fonsp]

Hey! Thanks for your contribution! Could you write a bit about who you are and what makes you interested in contributing to Pluto's accessibility? Do you have experience with Web Acccessibility?

-fons

[fonsp]

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