fix: a popup save no longer undoes a network switch the background saved during it (closes #641)

The popup's saveState() and the background's updateState() each read the
stored record, change it and write it back, each 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) from the read to
the write, through withLock() in src/shared/browserApi.js, and the recovery
screen's reset takes it too. A page in a Firefox private window gets locks of
its own and is not covered. 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

src/popup/index.js

@@ -220,7 +220,7 @@ async function widenApprovalWindow() {
 async function init() {
     // First, before anything can save: showView() saves on every navigation
     // without awaiting, so a save that fails from here on has somewhere to be
-    // reported rather than being swallowed by the save queue
+    // reported rather than reaching only the console
     // (https://git.eeqj.de/sneak/AutistMask/issues/362). Registered ahead of
     // the approval-window branch below too, since that window saves as well.
     //