feat: Add daemon single-instance lock and graceful shutdown to systemd service.

Core/DaemonService.cs

@@ -25,8 +25,22 @@ public static class DaemonService
         return Path.Combine(runtimeDir, "toak.sock");
     }
 
+    private static FileStream? _lockFile;
+
     public static async Task StartAsync(bool verbose)
     {
+        var lockPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), "toak", "daemon.lock");
+        try
+        {
+            Directory.CreateDirectory(Path.GetDirectoryName(lockPath)!);
+            _lockFile = new FileStream(lockPath, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);
+        }
+        catch (IOException)
+        {
+            Console.WriteLine("Toak daemon is already running.");
+            return;
+        }
+
         Logger.Verbose = verbose;
         var socketPath = GetSocketPath();