The popup's saveState() and the background's updateState() each read the
stored record, change it and write it back, and each was ordered only within
its own context, so a write by one could land between the other's read and
write and be lost. Both now hold the browser's lock (navigator.locks, shared
by every page and the background) from the read to the write, through
withLock() in src/shared/browserApi.js; the recovery screen's reset takes it
too. The lock also orders a page's own saves, so both per-context queues are
gone. The unit tests get a small lock from a Jest setup file.
Model: opus-5-5
