Merge icon, delete-branch-after-merge setting, floating tab bar; bump to 2.0

- Merge button: use the proper SF Symbol 'arrow.triangle.merge' instead of
  the download icon 'arrow.down.circle'.
- Add a Settings toggle 'Delete branch after merge'. When on, the merge call
  passes delete_branch_after_merge=true and the head branch is deleted after
  a successful merge (best-effort).
- PR detail bottom tab bar: floating capsule style (regularMaterial + shadow)
  like the latest iOS tab bars, instead of a full-width bar.
- Bump to 2.0.

GiteaPR/Detail/PRDetailView.swift

@@ -131,7 +131,9 @@ struct PRDetailView: View {
         .background(Color(.secondarySystemBackground))
     }
 
-    /// Bottom tab bar: icon-only navigation between the PR's sections.
+    /// Bottom tab bar: icon-only navigation between the PR's sections, in a
+    /// floating capsule style (like the latest iOS tab bars) rather than a
+    /// full-width bar.
     private var bottomTabBar: some View {
         HStack {
             ForEach(DetailTab.allCases) { tab in
@@ -149,8 +151,15 @@ struct PRDetailView: View {
                 }
             }
         }
-        .padding(.vertical, 6)
-        .background(.bar)
+        .padding(.vertical, 8)
+        .padding(.horizontal, 6)
+        .background(
+            Capsule()
+                .fill(.regularMaterial)
+                .shadow(color: .black.opacity(0.15), radius: 8, y: 2)
+        )
+        .padding(.horizontal, 16)
+        .padding(.bottom, 8)
     }
 
     private var reviewMenu: some View {
@@ -173,7 +182,7 @@ struct PRDetailView: View {
         Button {
             showMergeSheet = true
         } label: {
-            Image(systemName: "arrow.down.circle")
+            Image(systemName: "arrow.triangle.merge")
         }
         .disabled(vm.ciStatus == .failure &&
                   SettingsStore.shared.mergeGateMode == .block)