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

lib/widgets/custom_title_bar.dart

@@ -15,8 +15,20 @@ class CustomTitleBar extends StatelessWidget {
           height: 40,
           child: Stack(
             children: [
-              const Positioned.fill(
+              // Çok hafif degrade — masaüstü arka planı hafifçe karartır
-                child: SizedBox.shrink(),
+              Positioned.fill(
+                child: Container(
+                  decoration: const BoxDecoration(
+                    gradient: LinearGradient(
+                      begin: Alignment.topCenter,
+                      end: Alignment.bottomCenter,
+                      colors: [
+                        Color.fromRGBO(0, 0, 0, 0.1),
+                        Color.fromRGBO(0, 0, 0, 0.0),
+                      ],
+                    ),
+                  ),
+                ),
               ),
               Positioned(
                 right: 0,