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, 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 except a page in a Firefox private window)
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

tests/support/navigatorLocks.js

@@ -0,0 +1,26 @@
+// `navigator.locks` for the unit tests, which run on node, where there is none.
+//
+// One lock per name: granted in the order it was asked for, and let go when
+// the callback settles, whether it resolved or rejected. That is what the
+// browser's own does for the one call the extension makes,
+// `navigator.locks.request(name, callback)`. Every module a test file loads
+// shares it, as every page of the extension and its background share the
+// browser's.
+//
+// Run before each test file by `setupFiles` in package.json.
+
+// For each name, a promise that settles once the last request for it has
+// let the lock go.
+const released = new Map();
+
+function request(name, callback) {
+    const previous = released.get(name) || Promise.resolve();
+    const result = previous.then(() => callback({ name, mode: "exclusive" }));
+    released.set(
+        name,
+        result.catch(() => {}),
+    );
+    return result;
+}
+
+globalThis.navigator = { locks: { request } };