feat(wallet): IWalletAccount abstraction and WIF/xpub/xprv keystore import types

Introduce IWalletAccount interface to abstract HD, imported-xprv, WIF,
and watch-only account types. HdAccount implements the interface; the
new ImportedKeyAccount handles lists of WIF keys with fixed addresses
(no gap scanning, change returns to first address).

WalletLoader gains three factory methods:
- NewFromXpub: watch-only from SLIP-132 xpub/ypub/zpub
- NewFromXprv: spendable from SLIP-132 xprv/yprv/zprv
- NewFromWif: spendable from one or more WIF private keys

WalletDocument gains optional AccountXprv and WifKeys fields; AccountPath
and AccountXpub become nullable (absent for WIF wallets). IsWatchOnly
updated to cover all non-signing wallet kinds.

TransactionFactory and CLI OpenWallet() updated to IWalletAccount.
12 new unit tests in ImportedKeyAccountTests cover factory round-trips,
address derivation, and watch-only detection.

tests/PalladiumWallet.Tests/Wallet/WalletLoaderTests.cs

@@ -153,7 +153,8 @@ public class WalletLoaderTests
             AccountXpub = doc.AccountXpub,
         };
         var account = WalletLoader.ToAccount(docWo);
-        Assert.Throws<InvalidOperationException>(() => account.GetExtPrivateKey(false, 0));
+        Assert.True(account.IsWatchOnly);
+        Assert.Null(account.GetPrivateKey(false, 0));
     }
 
     [Fact]