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/shared/wallet.js

@@ -24,6 +24,25 @@ function hdWalletFromMnemonic(mnemonic) {
     return { xpub, firstAddress };
 }
 
+function hdWalletFromXprv(xprv) {
+    const node = HDNodeWallet.fromExtendedKey(xprv);
+    if (!node.privateKey) {
+        throw new Error("Not an extended private key (xprv).");
+    }
+    const xpub = node.neuter().extendedKey;
+    const firstAddress = node.deriveChild(0).address;
+    return { xpub, firstAddress };
+}
+
+function isValidXprv(key) {
+    try {
+        const node = HDNodeWallet.fromExtendedKey(key);
+        return !!node.privateKey;
+    } catch {
+        return false;
+    }
+}
+
 function addressFromPrivateKey(key) {
     const w = new Wallet(key);
     return w.address;
@@ -38,6 +57,10 @@ function getSignerForAddress(walletData, addrIndex, decryptedSecret) {
         );
         return node.deriveChild(addrIndex);
     }
+    if (walletData.type === "xprv") {
+        const node = HDNodeWallet.fromExtendedKey(decryptedSecret);
+        return node.deriveChild(addrIndex);
+    }
     return new Wallet(decryptedSecret);
 }
 
@@ -49,6 +72,8 @@ module.exports = {
     generateMnemonic,
     deriveAddressFromXpub,
     hdWalletFromMnemonic,
+    hdWalletFromXprv,
+    isValidXprv,
     addressFromPrivateKey,
     getSignerForAddress,
     isValidMnemonic,