[Bl0ck154]

Why

The current account and settings stores use direct file writes. Concurrent clients can overwrite one another's changes, and an interrupted write can leave a partial JSON file.

Changes

Adapt upstream PR Lampese#182 to our v0.2.21 fork:

• Cross-process locks around read-modify-write operations in the account and settings stores.
• Atomic replacement of accounts.json, settings.json and Codex auth.json using private temporary files and fsync.
• Keep our workspace-aware duplicate identity and healthy/stale OAuth re-login behavior, with the same preserved UUID, labels and timestamps.
• Preserve our upstream fix: preserve active account on add with OAuth command regression tests Lampese/codex-switcher#153 inactive-on-add semantics: adding the first account also does not switch live auth.
• Keep existing macOS settings and account management behavior.

Validation

• All merge conflicts manually resolved, and git diff --check passed.
• Rust builds/tests/format checks are delegated to the GitHub PR CI because this work was prepared on a production VPS that must not compile the application.
• Do not merge until CI passes and the OAuth/workspace regression tests are reviewed.

This is a separate branch from main, independent of the remote-dashboard security PR #11.
Original work: Lampese#182