Preserve sidebar footer disclosure state across route remounts

[bb-slop-cop]

Human comments

What was wrong

The managed sidebar-footer host kept its active disclosure key only in React state owned by AppSidebar. On wide settings and tools routes, the layout replaces that component, so returning mounted a fresh hook initialized with no active disclosure. The verified reproduction and root-cause report is https://get-bb.github.io/reports/issues/3245.html.

What changed

The existing sidebar-footer host now retains the active registration key across host remounts and updates it on open, close, toggle, and dismiss actions. A focused regression test covers persistence across a remount and confirms that an explicit close still clears the retained state. There are no wire, protocol, CLI, dependency, schema, generated-file, or stored-data changes.

How you verified

• The focused test failed on trusted origin/main because the remounted trigger had aria-expanded="false", then passed after the fix.
• pnpm exec turbo run test --filter=@bb/app -- src/components/plugin/PluginSidebarFooterItems.test.tsx src/components/plugin/PluginSidebarFooterItems.persistence.test.tsx src/components/layout/AppLayoutSidebar.test.tsx — 10 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/app — passed.
• Live isolated Chrome check — the disclosure stayed expanded after a settings round trip and collapsed after its explicit close control.
• pnpm exec oxfmt --check ... and git diff --check — passed.
• Changed text: 125 lines total (121 additions, 4 deletions) across 2 files.

Fixes #3245

AGENT GENERATED