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. The network it takes is not counted as the popup's own change,
even when that save fails to write. It then leaves the page as a switch
in Settings does. 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/trackedTokensPerNetwork.test.js

@@ -161,6 +161,7 @@ test("a token added by contract address on Sepolia is tracked on Sepolia only",
     );
     expect(stored().trackedTokens).toEqual([SEPOLIA_TOKEN]);
 
+    await boot.click("btn-settings");
     await chooseNetwork(boot, "mainnet");
     expect(stored().networkId).toBe("mainnet");
     expect(stored().trackedTokens).toEqual([MAINNET_TOKEN]);