feat(ios): tab tabanlı navigasyon ve okuma durumu takibi ekle

ios/Bookibra/DesignSystem/Theme.swift

@@ -26,4 +26,21 @@ enum Theme {
     static func headerSerif(size: CGFloat) -> Font {
         .custom("NewYork-Regular", size: size, relativeTo: .largeTitle)
     }
+
+    enum Spacing {
+        static let xSmall: CGFloat = 6
+        static let small: CGFloat = 10
+        static let medium: CGFloat = 16
+        static let large: CGFloat = 24
+    }
+
+    enum Radius {
+        static let card: CGFloat = 14
+        static let image: CGFloat = 10
+        static let pill: CGFloat = 999
+    }
+
+    enum Shadow {
+        static let card = (color: Color.black.opacity(0.14), radius: CGFloat(8), y: CGFloat(4))
+    }
 }