Tab-scoped sidebar persistence

[masnmt]

Currently, <flux:sidebar> stores its collapsed state in localStorage, so the state is shared across browser tabs.

It would be useful to have a tab-scoped persistence mode, for example:

<flux:sidebar collapsible persist="tab">

When persist="tab" is specified, it could prefer the value from sessionStorage, falling back to localStorage for a newly opened tab:

const collapsed =
    sessionStorage.getItem(key)
    ?? localStorage.getItem(key)

When the state changes, both could be updated:

sessionStorage.setItem(key, value)
localStorage.setItem(key, value)

This would keep the sidebar state independent between open tabs, while preserving the latest state as the default for newly opened tabs.