fix: the private key export screen opens again in the same session (closes #460)

show() found the address line through the element inside it, then
replaced the line's contents with renderAddressHtml(), which deleted
that element, so the next show() in the same popup session threw
before it navigated. The line now carries the export-privkey-address
id itself and is looked up by it. No other view that renders an
address finds its container through a child.

The jest DOM stub now takes an element out of the document when its
parent's contents are replaced, and a new test opens the screen
twice. The #253 e2e case no longer reopens the popup before its
second open.

Model: opus-5-5

src/popup/views/exportPrivkey.js

@@ -98,7 +98,7 @@ function show(walletIdx, addrIdx) {
 
     $("export-privkey-title").textContent =
         wallet.name + " — Address " + (addrIdx + 1);
-    const addrContainer = $("export-privkey-dot").parentElement;
+    const addrContainer = $("export-privkey-address");
     addrContainer.innerHTML = renderAddressHtml(addr.address);
     attachCopyHandlers(addrContainer);