Merge spinner + swipeable PR sections; bump to 3.1

- MergeSheet: show a spinner in the toolbar while the merge is confirming/
  rejecting, lock Cancel and interactive dismissal during the request, and
  prevent double-taps.
- PR detail: replace the ScrollView-switch with a page-style TabView so you
  can swipe left/right to change sections (iOS 26 style); bottom bar taps
  still select. Make DetailTab Hashable. Bump to 3.1.

GiteaPR/Detail/Sheets.swift

@@ -65,6 +65,7 @@ struct MergeSheet: View {
     @ObservedObject var vm: PRDetailViewModel
     @Environment(\.dismiss) private var dismiss
     @State private var method = "merge"
+    @State private var isMerging = false
     /// Called when the merge succeeds, so the parent can show the success
     /// animation and pop back to the landing page.
     var onSuccess: () -> Void = {}
@@ -118,20 +119,33 @@ struct MergeSheet: View {
             .navigationBarTitleDisplayMode(.inline)
             .toolbar {
                 ToolbarItem(placement: .cancellationAction) {
-                    Button("Cancel") { dismiss() }
+                    if isMerging {
+                        // No cancel while a merge is in flight.
+                    } else {
+                        Button("Cancel") { dismiss() }
+                    }
                 }
                 ToolbarItem(placement: .confirmationAction) {
-                    Button("Merge") { performMerge() }
-                        .disabled(SettingsStore.shared.mergeGateMode == .block
-                                  && vm.ciStatus == .failure)
+                    if isMerging {
+                        // Spinner while the merge is confirming/rejecting.
+                        ProgressView()
+                    } else {
+                        Button("Merge") { performMerge() }
+                            .disabled(SettingsStore.shared.mergeGateMode == .block
+                                      && vm.ciStatus == .failure)
+                    }
                 }
             }
+            .interactiveDismissDisabled(isMerging)
         }
     }
 
     private func performMerge() {
+        guard !isMerging else { return }
+        isMerging = true
         Task {
             let success = await vm.merge(method: method)
+            isMerging = false
             if success {
                 dismiss()
                 onSuccess()