Logout left lotus-per-user-volume (Element Call's per-person volume map:
whom the previous user called and turned up/down; same origin on the web)
and settings-sync-meta:<userId> (a key naming the account) in
localStorage. clearPlaintextCaches now removes both; device preferences
(`settings`) are kept as before.
Verified: after joining/leaving a call and logging out via Settings, both
keys are gone (unit test added).
Fixes#347
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5