ux: Seamless Timeline

This PR is a ux change to make the header, tabbar, and post button disappear when the user scrolls.
The main tabbar is now an overlay which means it will display over views, this was needed in
order to get the timeline to extend behind it. However, this mean we must add bottom padding to any
view where the main tabbar is present to account for the overlap.

Changelog-Added: Disappearing header, tabbar, and post button on scroll

Signed-off-by: ericholguin <ericholguin@apache.org>

damus/Util/Extensions/OffsetExtension.swift

@@ -0,0 +1,78 @@
+//
+//  OffsetExtension.swift
+//  damus
+//
+//  Created by eric on 9/6/24.
+//
+
+import SwiftUI
+
+enum SwipeDirection {
+    case up
+    case down
+    case none
+}
+
+extension View {
+    @ViewBuilder
+    func offsetY(completion: @escaping (CGFloat, CGFloat)->())->some View {
+        self
+            .modifier(OffsetHelper(onChange: completion))
+    }
+    
+    func safeArea() -> UIEdgeInsets {
+        guard let scene = this_app.connectedScenes.first as? UIWindowScene else{return .zero}
+        guard let safeArea = scene.windows.first?.safeAreaInsets else{return .zero}
+        return safeArea
+    }
+}
+
+struct OffsetHelper: ViewModifier{
+    var onChange: (CGFloat,CGFloat)->()
+    @State var currentOffset: CGFloat = 0
+    @State var previousOffset: CGFloat = 0
+    
+    func body(content: Content) -> some View {
+        content
+            .overlay {
+                GeometryReader{proxy in
+                    let minY = proxy.frame(in: .named("scroll")).minY
+                    Color.clear
+                        .preference(key: OffsetKey.self, value: minY)
+                        .onPreferenceChange(OffsetKey.self) { value in
+                            previousOffset = currentOffset
+                            currentOffset = value
+                            onChange(previousOffset,currentOffset)
+                        }
+                }
+            }
+    }
+}
+
+struct OffsetKey: PreferenceKey {
+    static var defaultValue: CGFloat = 0
+    
+    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
+        value = nextValue()
+    }
+}
+
+struct HeaderBoundsKey: PreferenceKey{
+    static var defaultValue: Anchor<CGRect>?
+    
+    static func reduce(value: inout Anchor<CGRect>?, nextValue: () -> Anchor<CGRect>?) {
+        value = nextValue()
+    }
+}
+
+func getSafeAreaTop()->CGFloat{
+    guard let scene = this_app.connectedScenes.first as? UIWindowScene else{return .zero}
+    guard let topSafeArea = scene.windows.first?.safeAreaInsets.top else{return .zero}
+    return topSafeArea
+}
+
+func getSafeAreaBottom()->CGFloat{
+    guard let scene = this_app.connectedScenes.first as? UIWindowScene else{return .zero}
+    guard let bottomSafeArea = scene.windows.first?.safeAreaInsets.bottom else{return .zero}
+    return bottomSafeArea
+}