Coming back to the feed shows it as it was left

Leaving the feed keeps it in Services/FeedMemory.cs (posts, waiting posts, cursors and scroll; ten minutes, for the same
feed of the same persona), so coming back from a thread or a profile shows it again at once, older pages included,
instead of reading it anew; newer posts come behind the "new" button and through the stream as before.
tests/e2e FeedMemoryTests goes to a thread and back without a new read of the home timeline; the whole suite passes
(123, 5 skipped).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Services/FeedMemory.cs

@@ -0,0 +1,51 @@
+using decePubClient.Models;
+using decePubClient.Models.Mastodon;
+
+using PrivaPub.ClientModels.Group;
+
+namespace decePubClient.Services
+{
+	/// <summary>
+	/// The feed as the reader left it: its query, posts, waiting posts, cursors and scroll. Coming back from a thread or a
+	/// profile shows it again at once instead of reading it anew; newer posts come behind the "new" button as usual. Kept
+	/// ten minutes, and only for the same feed of the same persona.
+	/// </summary>
+	public class FeedMemory
+	{
+		public static readonly TimeSpan Keep = TimeSpan.FromMinutes(10);
+
+		public FeedQuery Query { get; private set; }
+		public List<FeedPost> Posts { get; private set; } = [];
+		public List<FeedPost> Pending { get; private set; } = [];
+		public List<ViewGroup> Groups { get; private set; } = [];
+		public List<MastodonList> Lists { get; private set; } = [];
+		public string MinId { get; private set; }
+		public string MaxId { get; private set; }
+		public double ScrollTop { get; private set; }
+		public DateTime SavedAt { get; private set; }
+
+		public void Save(FeedQuery query, List<FeedPost> posts, List<FeedPost> pending, List<ViewGroup> groups, List<MastodonList> lists,
+			string minId, string maxId, double scrollTop)
+		{
+			Query = query;
+			Posts = posts;
+			Pending = pending;
+			Groups = groups;
+			Lists = lists;
+			MinId = minId;
+			MaxId = maxId;
+			ScrollTop = scrollTop;
+			SavedAt = DateTime.UtcNow;
+		}
+
+		/// <summary>Whether what is kept is recent and belongs to this persona.</summary>
+		public bool Holds(string personaId) => Query is not null && Query.PersonaId == personaId && DateTime.UtcNow - SavedAt < Keep;
+
+		public void Forget()
+		{
+			Query = null;
+			Posts = [];
+			Pending = [];
+		}
+	}
+}