perf(ui): reduce hidden session lifecycle overhead

[pascalandr]

Summary

• Lower hidden session cache pressure and unmount dormant panes sooner to reduce background UI cost.
• Preserve hidden session and settings state while batching perf trace persistence and related follow-up fixes.
• Stack this branch on ready/ui-locale-bootstrap so the review stays focused on hidden-session lifecycle behavior.

Dependencies

• Depends on perf(ui): defer locale bootstrap and overlay bundles #3
• Transitive upstream dependency: perf(tauri): instrument desktop startup NeuralNomadsAI/CodeNomad#216
• This PR stays on my fork until #3 is ready to move upstream.

[pascalandr]

Keeping this one in draft for now.

The stack is updated underneath it, but I would still like more soak time on the hidden session lifecycle changes before moving it out of draft.

[pascalandr]

Restacked this on top of the refreshed ready/ui-locale-bootstrap branch.

I am still keeping it in draft for now, but the stack underneath it is now up to date.

[pascalandr]

Closing this stale internal fork PR. The branch stack is no longer used as a live review path.