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

lib/widgets/custom_title_bar.dart

@@ -15,8 +15,20 @@ class CustomTitleBar extends StatelessWidget {
           height: 40,
           child: Stack(
             children: [
-              const Positioned.fill(
-                child: SizedBox.shrink(),
+              // Çok hafif degrade — masaüstü arka planı hafifçe karartır
+              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,