fix: an open popup follows a network switch the background saves (closes #650)

When the background saves a network switch after the user approves a
site's request, or another popup page saves one, an open popup takes the
new network from the stored record inside a save, holding the lock, and
checks the record once at start-up for a switch saved before it
listened. It then leaves the page as a switch in Settings does: the cut
and the redraw are two Settings functions both paths call, and both
reset the popup's caches through one function. A switch in Settings is
now made inside its own save too, so one picked while a follow waits for
the lock is made after it. The popup never switches the network itself.

Model: opus-5-5

tests/balanceRefreshNetworkSwitch.test.js

@@ -97,9 +97,9 @@ test("a refresh that returns after a switch in Settings writes nothing, and the
     );
 });
 
-// The stored record still holds Ethereum Mainnet while the refresh's save reads
-// it, so only this page's own network shows the switch.
-test("a refresh whose save is reading the stored record when the user switches in Settings writes nothing", async () => {
+// The switch is made only once the refresh's save, holding the lock, has
+// written Ethereum Mainnet's balances, and the switch then clears them.
+test("a refresh whose save is reading the stored record when the user switches in Settings leaves none of its balances stored", async () => {
     const page = await bootPopup(unversionedValidProfile());
     expect(page.pageErrors).toEqual([]);
     const { state } = require("../src/shared/state");
@@ -145,7 +145,8 @@ test("a refresh that returns after the background switched the network writes no
 
     const refresh = await holdRefresh(page);
     // The background writes the switch to Sepolia, as its updateState() does
-    // once the user approves a site's request. This page is not told.
+    // once the user approves a site's request. This page is not told of it
+    // yet.
     const record = page.storage.read("autistmask");
     applyChainSwitchFields(record, "sepolia");
     page.storage.write("autistmask", record);