Replace PendingAuth/RSA key passing with tempPassword pattern:
- register(): saves keys to disk, stores password temporarily
- confirmRegistration(): verifies code, then calls performLogin(email, tempPassword)
which re-loads keys from disk - exactly as iOS does
- Extract shared performLogin() suspend fn used by both login() and post-confirm flow
- Remove PendingAuth data class entirely
Simpler, correct, matches reference iOS implementation.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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