feat: title bar hafif degrade (alpha 0.1) + üst bölge masaüstüne saydam

lib/screens/viewer_screen.dart

@@ -118,7 +118,13 @@ class _ViewerScreenState extends State<ViewerScreen> with WindowListener {
             },
             child: Stack(
               children: [
-                Container(color: const Color(0xFF1a1a1a)),
+                Positioned(
+                  top: 40,
+                  left: 0,
+                  right: 0,
+                  bottom: 0,
+                  child: Container(color: const Color(0xFF1a1a1a)),
+                ),
                 Container(
                   color: Colors.transparent,
                   child: _hasImages