fix: the delete wallet screens name the wallet's own key material (closes #529)

DeleteWallet and DeleteWalletLostPassword told key and xprv wallets that
their recovery phrase was the way back, but those wallets have none. Both
screens now name the recovery phrase, the private key or the extended
private key, according to the wallet's kind.

The noun comes from keyMaterialName() in the popup view helpers, which
DeleteAddress now uses too in place of its own two-way choice, so these
three screens get it from one place. The lost-password screen's route-back
sentence is reworded to fit all three kinds.

Model: opus-5-5

src/popup/views/helpers.js

@@ -419,10 +419,10 @@ function addressHoldsFunds(addr) {
     return false;
 }
 
-// What a wallet was imported from, which is also its only backup: the
-// recovery phrase, the private key or the extended private key. The screens
-// that tell the user what to keep or import again name it through here, so
-// a key or xprv wallet is never sent looking for a recovery phrase.
+// The name of a wallet's key material, which is also its only backup: the
+// recovery phrase of an HD wallet, the private key of a key wallet, the
+// extended private key of an xprv wallet. DeleteWallet,
+// DeleteWalletLostPassword and DeleteAddress name the backup with it.
 function keyMaterialName(wallet) {
     if (walletHasRecoveryPhrase(wallet)) return "recovery phrase";
     if (wallet.type === "key") return "private key";