Fix compile errors: guard unwrap + ViewBuilder var mutation

- PRDetailView: guard let current = selectedTab ?? .overview fails because
  ?? already unwraps to non-optional. Use let current = selectedTab ?? .overview.
- MarkdownText: var inCodeBlock inside ViewBuilder is not allowed. Precompute
  lines with their code-block state as a computed property on a private struct,
  then iterate via ForEach in the builder. Bump to 3.3.

GiteaPR/Detail/PRDetailView.swift

@@ -195,8 +195,8 @@ struct PRDetailView: View {
             DragGesture(minimumDistance: 20)
                 .onEnded { value in
                     let tabs = DetailTab.allCases
-                    guard let current = selectedTab ?? .overview,
-                          let idx = tabs.firstIndex(of: current) else { return }
+                    let current = selectedTab ?? .overview
+                    guard let idx = tabs.firstIndex(of: current) else { return }
                     if value.translation.width < -40, idx < tabs.count - 1 {
                         withAnimation { selectedTab = tabs[idx + 1] }
                     } else if value.translation.width > 40, idx > 0 {