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

README.md

@@ -461,13 +461,15 @@ address, the transaction assertions run against the raw signed transaction
 captured at `eth_sendRawTransaction` rather than against anything the extension
 reported, rejecting each prompt is required to return a rejection to the page
 rather than hang or resolve, a network switch request is required to leave the
-stored network unchanged and send no `chainChanged` until it is approved, a
-network switch in Settings is required to send the page `chainChanged` with the
-new chain id, a prompt raised while another approval window has focus is
-required to open a window of its own, and the password is required to be absent
-from every message the approval window sends to the background — with the
-message that would carry it required to be present, so that check cannot pass by
-observing nothing. That last one is the standing floor under
+stored network unchanged and send no `chainChanged` until it is approved, the
+popup open all along is required to show the `[TESTNET]` banner once a switch to
+Sepolia is approved and to drop it once the switch back is, a network switch in
+Settings is required to send the page `chainChanged` with the new chain id, a
+prompt raised while another approval window has focus is required to open a
+window of its own, and the password is required to be absent from every message
+the approval window sends to the background — with the message that would carry
+it required to be present, so that check cannot pass by observing nothing. That
+last one is the standing floor under
 [#157](https://git.eeqj.de/sneak/AutistMask/issues/157).
 
 The limits of that coverage and of the rest of the Chrome suite, none of them
@@ -603,9 +605,10 @@ suite), wallet creation through the UI, the Add Token screen, and the dApp round
 trips — `eth_requestAccounts`, `personal_sign`, `wallet_switchEthereumChain`
 rejected and then approved, `eth_sendTransaction`, and a closed approval window
 rejecting with EIP-1193 4001 — driven through the real content script,
-background page and approval windows. The `personal_sign` prompt is also
-required to fit its window without scrolling sideways, with the extension's
-pages at 170% zoom.
+background page and approval windows. The popup open all along is required to
+show the `[TESTNET]` banner once the switch to Sepolia is approved and to drop
+it once the switch back is. The `personal_sign` prompt is also required to fit
+its window without scrolling sideways, with the extension's pages at 170% zoom.
 
 The suite lives in `tests/e2e/firefox/`. Its WebDriver client (`driver.js`) has
 **no npm dependencies at all**: it is built on global `fetch` and
@@ -1564,20 +1567,29 @@ such as an edit typed into Settings and not yet saved. Home is the one screen
 "Back" always re-renders, so the wallet list reflects anything that changed
 while the user was away from it.
 
-A network switch in Settings leaves every screen beneath Settings showing the
-previous network, so after it "Back" behaves as in a reopened popup: every
-screen on the stack counts as not yet rendered, and the stack is cut at the
-first screen that cannot be rendered from saved state, such as Send or AddToken,
-or at AddressToken for any token but the native one, because that token was
-picked on the previous network. The cut stack is saved with the switch, so a
-popup closed straight after it and reopened onto Settings lands in the same
-place. "Back" then renders the screen it lands on for the new network, and
-anything typed there and not saved, which belonged to the previous network, is
-dropped. ConfirmTx, WaitTx, SuccessTx, ErrorTx and TransactionDetail are
-rendered instead for the network of the transaction they show: the network it
-was reviewed on for ConfirmTx, and the network it was sent on for the others.
-Without a wallet the stack is not cut, because it starts at Welcome, which shows
-nothing from a network.
+A network switch leaves every screen on the page showing the previous network.
+The user makes it in Settings, or approves a site's request and the background
+saves it. Any other popup open meanwhile takes the switch from the stored
+record, even one saved while it was opening, and never switches the network
+itself. A switch the user picks in its Settings while it takes one is made after
+it, not overwritten by it. An approval window does not take the switch. After
+the switch, whichever way the page got it, "Back" behaves as in a reopened
+popup: every screen on the stack counts as not yet rendered, and the stack is
+cut at the first screen that cannot be rendered from saved state, such as Send
+or AddToken, or at AddressToken for any token but the native one, because that
+token was picked on the previous network and is no longer selected. The cut
+stack is saved with the popup's new network, so a popup closed straight after
+and reopened lands in the same place. The screen on top is drawn again for the
+new network: Settings in place, and any other screen as a reopened popup draws
+it, so Home takes the place of one that cannot be rendered from saved state or
+of AddressToken for a token no longer selected. "Back" then renders the screen
+it lands on for the new network, and anything typed there and not saved, which
+belonged to the previous network, is dropped. ConfirmTx, WaitTx, SuccessTx,
+ErrorTx and TransactionDetail are rendered instead for the network of the
+transaction they show: the network it was reviewed on for ConfirmTx, and the
+network it was sent on for the others. Without a wallet the stack is not cut and
+only Settings is drawn again, because the stack starts at Welcome, and Welcome
+and AddWallet show nothing from a network.
 
 Every screen that holds secret material in the page registers a cleanup with
 `onViewLeave()` (`src/popup/views/helpers.js`), which `showView()` runs on every