Fix compile: scrollPosition needs optional binding; selectedTab is now DetailTab?

scrollPosition(id:) requires Binding<some Hashable?>, but selectedTab was a
non-optional DetailTab — the build failed with 'generic parameter some
Hashable could not be inferred'. Make selectedTab optional (DetailTab? =
.overview); bottom-bar == comparisons still work via optional promotion.

GiteaPR/Detail/PRDetailView.swift

@@ -3,7 +3,7 @@ import SwiftUI
 struct PRDetailView: View {
     @StateObject private var vm: PRDetailViewModel
     @Environment(\.dismiss) private var dismiss
-    @State private var selectedTab: DetailTab = .overview
+    @State private var selectedTab: DetailTab? = .overview
     @State private var showCommentSheet = false
     @State private var showMergeSheet = false
     @State private var showCloseConfirm = false