perf(ui): defer locale bootstrap and overlay bundles

[pascalandr]

Summary

• Defer locale and overlay bundles so startup can mount before secondary UI payloads finish loading.
• Persist a small bootstrap cache for theme and locale hints while trimming hidden bootstrap and session work.
• Fall back cleanly to English if a lazy locale chunk fails so startup still mounts.

Dependencies

• Depends on perf(tauri): instrument desktop startup NeuralNomadsAI/CodeNomad#216
• This PR stays on my fork until #216 lands upstream.

[pascalandr]

This is now ready for review.

Its base branch now targets dev, and this follow-up still stacks cleanly on top of the refreshed startup instrumentation branch.

[pascalandr]

Restacked this on top of the rebuilt ready/tauri-startup-instrumentation branch.

This follow-up now sits on the refreshed 216 base and remains ready for review.