feat: add xprv wallet import support

Add the ability to import an existing HD wallet using an extended
private key (xprv) instead of a mnemonic phrase.

- New 'xprv' wallet type with full HD derivation and address scanning
- New importXprv view with password encryption
- Updated getSignerForAddress to handle xprv wallet type
- Added xprv link to the add-wallet view
- Allow adding derived addresses for xprv wallets

Closes #20

src/popup/views/addWallet.js

@@ -124,6 +124,11 @@ function init(ctx) {
         "click",
         ctx.showImportKeyView,
     );
+
+    $("btn-add-wallet-import-xprv").addEventListener(
+        "click",
+        ctx.showImportXprvView,
+    );
 }
 
 module.exports = { init, show };