Implement mark all as read in feed context menu for timeline

Multiplatform/Shared/Timeline/TimelineContextMenu.swift

@@ -81,6 +81,18 @@ struct TimelineContextMenu: View {
 			}
 		}
 		
+		if let feed = timelineItem.article.webFeed, timelineModel.canMarkAllAsReadInFeed(feed) {
+			Divider()
+			Button {
+				timelineModel.markAllAsReadInFeed(feed)
+			} label: {
+				Text("Mark All as Read in “\(feed.nameForDisplay)”")
+				#if os(iOS)
+				AppAssets.markAllAsReadImage
+				#endif
+			}
+		}
+
 		if timelineModel.canOpenIndicatedArticleInBrowser(timelineItem.article) {
 			Divider()
 			Button {

Multiplatform/Shared/Timeline/TimelineModel.swift

@@ -189,6 +189,19 @@ class TimelineModel: ObservableObject, UndoableCommandRunner {
 		markArticlesWithUndo(articlesToMark, statusKey: .read, flag: true)
 	}
 	
+	func canMarkAllAsReadInFeed(_ feed: Feed) -> Bool {
+		guard let articlesSet = try? feed.fetchArticles() else {
+			return false
+		}
+		return Array(articlesSet).canMarkAllAsRead()
+	}
+	
+	func markAllAsReadInFeed(_ feed: Feed) {
+		guard let articlesSet = try? feed.fetchArticles() else { return	}
+		let articlesToMark = Array(articlesSet)
+		markArticlesWithUndo(articlesToMark, statusKey: .read, flag: true)
+	}
+	
 	func toggleStarredStatusForSelectedArticles() {
 		guard !selectedArticles.isEmpty else {
 			return