fix(desktop): continue recovered machine identity setup

[RossHartmann]

Summary

• routes the recovered native identity directly into machine setup instead of the newly-created-key backup screen
• preserves correct Back navigation for that existing-identity path
• updates the existing navigation E2E to exercise the continuity path
• classifies the touched upstream test and updates the measured fork footprint
• mirrors the provider-neutral fix in fix(desktop): preserve existing identity onboarding context block/buzz#6552

Focused verification

• machine-onboarding helper tests: 9/9 green
• focused Biome check: green
• fork replay/footprint guard: 83/83 classified, zero Kiingo business logic in upstream production source, zero contamination
• diff check: green
• protected CI is rerunning the complete desktop matrix