fix: serialize Android sign-in and sign-out lifecycle

[BeforeLights]

Summary

• serialize complete sign-in/sign-out operations per account/workspace scope
• prevent key deletion and revocation from interleaving with key initialization
• add a blocking JVM regression test and record the automatic promotion-review disposition

Verification

• ./gradlew.bat :app:testDebugUnitTest --offline
• corepack pnpm test
• git diff --check

Review-flow note

This is a focused promotion-gate follow-up for the automatic CodeRabbit review of PR #14. It is intentionally a small exception to the normal 30–70 commit batching rule. CodeRabbit is not being invoked on this dev PR.

[coderabbitai]

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