Add a Page tab that keeps the last shown page reachable

mac/Sources/Byre/Sheet.swift

@@ -5,7 +5,6 @@ enum SheetView {
     case tab(TaskTab)
     case image(NSImage)
     case diagram(BoxLayout)
-    case page(Page)
 }
 
 struct Sheet {
@@ -82,7 +81,7 @@ private struct SheetPanel: View {
                     .lineLimit(1)
                     .truncationMode(.tail)
                 Spacer(minLength: 0)
-                if case .page(let page) = sheet.view, let url = page.url(store) {
+                if sheet.tab == .page, let page = store.selectedTaskId.flatMap({ store.shownPages[$0] }), let url = page.url(store) {
                     Button {
                         NSWorkspace.shared.open(url)
                     } label: {
@@ -129,7 +128,6 @@ private struct SheetPanel: View {
         case .tab(let t): tabLabels[t]!
         case .image: "Image"
         case .diagram: "Diagram"
-        case .page(let page): page.title(store)
         }
         if !overLeft {
             return "Lead · \(label)"
@@ -159,8 +157,6 @@ private struct SheetPanel: View {
             DiagramViewer(layout: layout)
                 .id(layout.id)
                 .padding(12)
-        case .page(let page):
-            PageView(page: page)
         }
     }
 }