fix(storage): acquire wallet lock before load and before CloseWallet

Three improvements to WalletLock integration:

1. OpenExisting acquires the lock before decrypting the file — no point
   attempting a potentially expensive PBKDF2 derivation for a wallet
   already held by another instance.

2. OpenFromPath acquires the lock before CloseWallet — if the new wallet
   is unavailable, the current session stays intact instead of being
   silently destroyed.

3. OpenLoaded now receives an already-acquired WalletLock from the caller
   instead of acquiring it internally; TryAcquireWalletLock is removed.
   Callers that fail to open (wrong password, I/O error) dispose the lock
   in their catch blocks.

Also fix CoinAmount.Of() to throw ArgumentException for unknown units
instead of silently falling back to PLM.

src/App/ViewModels/MainWindowViewModel.cs

@@ -874,7 +874,9 @@ public partial class MainWindowViewModel : ViewModelBase
             for (var n = 2; WalletStore.Exists(path); n++)
                 path = Path.Combine(AppPaths.WalletsDir(Net), $"wallet-{n}.wallet.json");
             WalletStore.Save(doc, path, password);
-            OpenLoaded(doc, account, path, password);
+            var newLock = WalletLock.TryAcquire(path);
+            if (newLock is null) { StatusMessage = Loc.Tr("msg.wallet.locked"); return; }
+            OpenLoaded(doc, account, path, password, newLock);
         }
         catch (Exception ex)
         {
@@ -885,24 +887,33 @@ public partial class MainWindowViewModel : ViewModelBase
     [RelayCommand]
     private void OpenExisting()
     {
+        var path = _pendingOpenPath ?? AppPaths.DefaultWalletPath(Net);
+        var password = string.IsNullOrEmpty(PasswordInput) ? null : PasswordInput;
+
+        // Il lock viene acquisito prima del caricamento: se il wallet è già aperto
+        // altrove, non ha senso nemmeno tentare la decifrazione.
+        var newLock = WalletLock.TryAcquire(path);
+        if (newLock is null) { StatusMessage = Loc.Tr("msg.wallet.locked"); return; }
+
         try
         {
-            var path = _pendingOpenPath ?? AppPaths.DefaultWalletPath(Net);
-            var password = string.IsNullOrEmpty(PasswordInput) ? null : PasswordInput;
             var doc = WalletStore.Load(path, password);
             _pendingOpenPath = null;
-            OpenLoaded(doc, WalletLoader.ToAccount(doc), path, password);
+            OpenLoaded(doc, WalletLoader.ToAccount(doc), path, password, newLock);
         }
         catch (WrongPasswordException)
         {
+            newLock.Dispose();
             StatusMessage = Loc.Tr("msg.wrongpassword");
         }
         catch (UnauthorizedAccessException)
         {
+            newLock.Dispose();
             StatusMessage = Loc.Tr("msg.wallet.noaccess");
         }
         catch (Exception ex)
         {
+            newLock.Dispose();
             StatusMessage = $"Errore: {ex.Message}";
         }
     }
@@ -910,16 +921,22 @@ public partial class MainWindowViewModel : ViewModelBase
     /// <summary>Apertura di un file wallet qualunque (menu File → Apri, multi-wallet §8).</summary>
     public void OpenFromPath(string path)
     {
+        // Il lock viene acquisito prima di chiudere l'eventuale wallet aperto:
+        // se il nuovo wallet non è disponibile, la sessione corrente resta intatta.
+        var newLock = WalletLock.TryAcquire(path);
+        if (newLock is null) { StatusMessage = Loc.Tr("msg.wallet.locked"); return; }
+
         try
         {
+            var doc = WalletStore.Load(path);
             if (IsWalletOpen)
                 CloseWallet();
-            var doc = WalletStore.Load(path);
-            OpenLoaded(doc, WalletLoader.ToAccount(doc), path, password: null);
+            OpenLoaded(doc, WalletLoader.ToAccount(doc), path, password: null, newLock);
         }
         catch (WrongPasswordException)
         {
             // Cifrato: si chiede la password nel passo di apertura del wizard.
+            newLock.Dispose();
             if (IsWalletOpen)
                 CloseWallet();
             _pendingOpenPath = path;
@@ -930,10 +947,12 @@ public partial class MainWindowViewModel : ViewModelBase
         }
         catch (UnauthorizedAccessException)
         {
+            newLock.Dispose();
             StatusMessage = Loc.Tr("msg.wallet.noaccess");
         }
         catch (Exception ex)
         {
+            newLock.Dispose();
             StatusMessage = $"Errore: {ex.Message}";
         }
     }
@@ -948,23 +967,10 @@ public partial class MainWindowViewModel : ViewModelBase
         StatusMessage = "";
     }
 
-    private bool TryAcquireWalletLock(string path)
+    private void OpenLoaded(WalletDocument doc, HdAccount account, string path, string? password, WalletLock walletLock)
     {
-        var acquired = WalletLock.TryAcquire(path);
-        if (acquired is null)
-        {
-            StatusMessage = Loc.Tr("msg.wallet.locked");
-            return false;
-        }
         _walletLock?.Dispose();
-        _walletLock = acquired;
-        return true;
-    }
-
-    private void OpenLoaded(WalletDocument doc, HdAccount account, string path, string? password)
-    {
-        if (!TryAcquireWalletLock(path))
-            return;
+        _walletLock = walletLock;
 
         // La rete del wallet comanda (registry, pin TLS, indirizzi).
         SelectedNetwork = doc.Network;

src/Core/Wallet/CoinAmount.cs

@@ -16,10 +16,11 @@ public static class CoinAmount
     /// <summary>(satoshi per unità, decimali mostrati) di ciascuna unità.</summary>
     private static (long Factor, int Decimals) Of(string unit) => unit switch
     {
+        "PLM"  => (SatsPerCoin, 8),
         "mPLM" => (100_000, 5),
         "µPLM" => (100, 2),
-        "sat" => (1, 0),
-        _ => (SatsPerCoin, 8), // PLM
+        "sat"  => (1, 0),
+        _ => throw new ArgumentException($"Unknown coin unit: {unit}", nameof(unit)),
     };
 
     public static string Format(long sats, string unit = "") =>