feat(watcher): add GetLastInputInfo P/Invoke for idle detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

WindowWatcher/NativeMethods.cs

@@ -13,4 +13,22 @@ internal static partial class NativeMethods
 
     [LibraryImport("user32.dll", SetLastError = true)]
     internal static partial uint GetWindowThreadProcessId(IntPtr hWnd, out uint processId);
+
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct LASTINPUTINFO
+    {
+        public uint cbSize;
+        public uint dwTime;
+    }
+
+    [DllImport("user32.dll")]
+    internal static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);
+
+    internal static TimeSpan GetIdleTime()
+    {
+        var info = new LASTINPUTINFO { cbSize = (uint)Marshal.SizeOf<LASTINPUTINFO>() };
+        if (!GetLastInputInfo(ref info))
+            return TimeSpan.Zero;
+        return TimeSpan.FromMilliseconds(Environment.TickCount64 - info.dwTime);
+    }
 }