docs: add account deletion e2e QA plan

[jjoonleo]

Advances #458.
References parent track #464.

Summary

• Add an issue-scoped end-to-end account deletion QA plan and evidence form.
• Add a handoff note documenting that [Release] QA account deletion end to end #458 remains externally blocked.
• Keep app behavior unchanged because backend verification and release-build prerequisites are still open.

Verification

• git diff --check -- plans/issue-458-account-deletion-e2e-qa.md handoff/issue-458-account-deletion-e2e-qa.md
• rg -n "#458|#439|#452|Evidence Form|Stop Conditions|Next Action" plans/issue-458-account-deletion-e2e-qa.md handoff/issue-458-account-deletion-e2e-qa.md

Remaining human tasks

• Complete [Release] Verify backend account and data deletion behavior #439: backend owner verifies deletion and retention behavior for each release-supported provider.
• Complete [Release] Build a signed release Android App Bundle #452: provide a signed release build or approved release-equivalent install path.
• Run the [Release] QA account deletion end to end #458 QA plan against each release-supported provider path and record evidence.
• Close [Release] QA account deletion end to end #458 only after the evidence shows account deletion, session invalidation, data handling, and failure recovery pass.