Allow sidebar for instance be collapsible

[SteKoe]

As described in #4912, the sidebar in SBA should be manually collapsible to allow the content to gain more space. Even though it collapses on mobile devices already, the functionality should be available at any time and trigger on small screens automatically.

[hiSandog]

Since the mobile auto-collapse already exists (#4912), the manual toggle can reuse that collapse component and just expose it on desktop widths. Three small things that would round it out: persist the collapsed state (localStorage or a setting) so it survives reloads, auto-collapse below a breakpoint reusing the mobile behavior, and bind a keyboard toggle (e.g. [ or a configurable key) for accessibility. Reusing the existing mobile collapse path keeps the change small and avoids maintaining a second sidebar-state machine. Happy to prototype if there's interest.