test: cover every control that refuses a defective wallet (closes #254)

Each control that leads to a signature or to the private key now has a
test that it refuses a defective wallet before decrypting anything: Send
on the main, address and token screens, Export Private Key, and both
approval screens, as drawn and as clicked.

Send on the confirmation screen had no such check. The Send buttons
stand in front of it, but the popup reopens onto it from a saved view,
so it now refuses the same way.

The comments that said the wallet's key cannot be derived now say that
getSignerForAddress refuses it, and the walletDefects module comment
names both earlier import paths.

Model: opus-5-5

src/popup/views/addressDetail.js

@@ -33,8 +33,8 @@ const { walletDefect } = require("../../shared/walletDefects");
 
 // The defect of the wallet the selected address belongs to, or null. Both the
 // send and the private-key export path check it before asking for a password,
-// so a wallet that cannot derive its keys says so instead of failing after the
-// user has typed one in.
+// so a wallet whose key getSignerForAddress refuses says so instead of failing
+// after the user has typed one in.
 function selectedWalletDefect() {
     if (state.selectedWallet === null) return null;
     return walletDefect(state.wallets[state.selectedWallet]);
@@ -259,9 +259,10 @@ function init(_ctx) {
     $("btn-export-privkey").addEventListener("click", () => {
         moreDropdown.classList.add("hidden");
         moreBtn.classList.remove("bg-fg", "text-bg");
-        // There is no private key to export for an address this wallet
-        // cannot derive. Without this the export screen would take a
-        // password and then report it as wrong.
+        // This address's private key can be derived from the stored key,
+        // but export goes through getSignerForAddress, which refuses a key
+        // that is not a master key. Without this the export screen would
+        // take a password and then report that refusal as a wrong password.
         const defect = selectedWalletDefect();
         if (defect) {
             showFlash(defect.shortMessage);