Add desktop diagnostics and resilient message handling

WirelessTextSyncer.Windows/Services/AppLogger.cs

@@ -0,0 +1,32 @@
+namespace WirelessTextSyncer.Windows.Services;
+
+public static class AppLogger
+{
+    private static readonly object SyncRoot = new();
+    private static readonly string LogDirectory = Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+        "WirelessTextSyncer");
+
+    public static string LogPath => Path.Combine(LogDirectory, "desktop.log");
+
+    public static void Info(string message)
+    {
+        Write("INFO", message);
+    }
+
+    public static void Error(string message, Exception? exception = null)
+    {
+        Write("ERROR", exception is null ? message : $"{message}{Environment.NewLine}{exception}");
+    }
+
+    private static void Write(string level, string message)
+    {
+        lock (SyncRoot)
+        {
+            Directory.CreateDirectory(LogDirectory);
+            File.AppendAllText(
+                LogPath,
+                $"{DateTimeOffset.Now:yyyy-MM-dd HH:mm:ss.fff zzz} [{level}] {message}{Environment.NewLine}");
+        }
+    }
+}