feat(storage): add WalletLock to prevent concurrent wallet access

Introduces an exclusive file lock (FileShare.None on wallet.json.lock)
held for the duration of a wallet session. A second instance trying to
open the same file receives null from TryAcquire and sees a localized
error message; UnauthorizedAccessException (OS permission issues)
propagates separately so the UI can show a distinct message.
The .lock file is deleted on Dispose (best-effort); the real guard is
the open FileStream, which the OS releases automatically on crash.

src/Core/Storage/WalletLock.cs

@@ -0,0 +1,48 @@
+namespace PalladiumWallet.Core.Storage;
+
+/// <summary>
+/// Exclusive lock on a wallet file held for the lifetime of the session.
+/// The real lock is the open FileStream with FileShare.None — the .lock file
+/// is just its vessel. The OS releases it automatically on process exit or crash.
+/// </summary>
+public sealed class WalletLock : IDisposable
+{
+    private readonly string _lockPath;
+    private FileStream? _stream;
+
+    private WalletLock(string lockPath, FileStream stream)
+    {
+        _lockPath = lockPath;
+        _stream = stream;
+    }
+
+    /// <summary>
+    /// Tries to acquire an exclusive lock for <paramref name="walletPath"/>.
+    /// Returns null if another process already holds the lock (IOException).
+    /// Lets UnauthorizedAccessException propagate so callers can show a distinct message.
+    /// </summary>
+    public static WalletLock? TryAcquire(string walletPath)
+    {
+        var lockPath = walletPath + ".lock";
+        try
+        {
+            var stream = new FileStream(
+                lockPath,
+                FileMode.OpenOrCreate,
+                FileAccess.ReadWrite,
+                FileShare.None);
+            return new WalletLock(lockPath, stream);
+        }
+        catch (IOException)
+        {
+            return null;
+        }
+    }
+
+    public void Dispose()
+    {
+        _stream?.Dispose();
+        _stream = null;
+        try { File.Delete(_lockPath); } catch { }
+    }
+}