Improve SwipeToDismiss modifier UX

This commit adds an opacity transition when swiping to dismiss an item,
to make it clear that the user is about to dismiss it.

Changelog-Changed: Swipe to dismiss on full screen carousel now shows an opacity effect for improved UX
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Modifiers/SwipeToDismiss.swift

@@ -12,11 +12,15 @@ struct SwipeToDismissModifier: ViewModifier {
     var onDismiss: () -> Void
     @State private var offset: CGSize = .zero
     @GestureState private var viewOffset: CGSize = .zero
+    
+    let threshold_offset: CGFloat = 100.0
+    let minimum_opacity: CGFloat = 0.1
 
     func body(content: Content) -> some View {
         content
             .offset(y: viewOffset.height)
             .animation(.interactiveSpring(), value: viewOffset)
+            .opacity(max(min(1.0 - (abs(offset.height) / threshold_offset), 1.0), minimum_opacity))
             .simultaneousGesture(
                 DragGesture(minimumDistance: minDistance ?? 10)
                     .updating($viewOffset, body: { value, gestureState, transaction in
@@ -28,7 +32,7 @@ struct SwipeToDismissModifier: ViewModifier {
                         }
                     }
                     .onEnded { _ in
-                        if abs(offset.height) > 100 {
+                        if abs(offset.height) > threshold_offset {
                             onDismiss()
                         } else {
                             offset = .zero