fix: activate first account after purge

[Loongphy]

Summary

• activate the first rebuilt account after purge when the current auth cannot be recovered
• reuse the normal activate path so auth.json backup behavior stays consistent
• add regression coverage and document the fallback activation behavior

Testing

• zig build test
• zig build run -- list

Note

Activate first account after purge when no active account exists

When purgeRegistryFromImportSource rebuilds accounts but none is marked active, it now calls activateAccountByKey on the first account in sorted order before saving the registry. This ensures a usable active account after purge instead of leaving the registry in an unauthenticated state. New unit and e2e tests cover the case where a stale auth.json is present and two accounts are rebuilt.

^{Macroscope summarized 1b61706.}

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commit: 1b61706