fix(desktop): preserve existing identity onboarding context

[RossHartmann]

Summary

• distinguish upgrade/recovery onboarding from a blank first launch using existing community state
• present the already-loaded native identity as Continue setup instead of implying that a new key will be created
• preserve the existing security boundary: legacy or mismatched community state still cannot auto-complete onboarding

Validation

• focused machine onboarding tests (9/9)
• focused Biome check on the four changed files
• git diff --check

[RossHartmann]

The generic four-file continuity fix is also submitted upstream as block#6552 so this fork delta has a removal path.