Fence the instance browser's first load, not only its next page

Pagination carried the login it was requested for, but the load that
fills the tab in the first place did not. Its subscriptions, feed and
playlist requests all resolve well after the credential behind them can
have been replaced, and cancelling the previous task does not settle it:
a request that already came back races the cancellation and installs the
previous person's private videos over the purge that was meant to remove
them. Popular and trending stay unfenced — they are unauthenticated, and
discarding them on a change of login would cost a fetch for nothing.

Discarding the account-scoped content now also forgets how far the feed
had been read. The high-water mark belongs to the feed that was thrown
away; left behind, it gates pagination against a page the next login
never loaded, and that login cannot ask for a second page until its
first exceeds a count it knows nothing about.

Yattee/Views/Instances/InstanceBrowseView.swift

@@ -1208,6 +1208,11 @@ struct InstanceBrowseView: View {
         feedPage = 1
         hasMoreFeedResults = false
         isLoadingMoreFeed = false
+        // The high-water mark belongs to the discarded feed. Left behind, it
+        // gates pagination against the previous login's count: the next one
+        // would have to exceed a page it never loaded before it could ask for
+        // a second.
+        feedLoadedVideoCount = 0
     }
 
     private func refreshLoginState() {
@@ -1253,6 +1258,13 @@ struct InstanceBrowseView: View {
         isLoading = !hasData  // Only show loading spinner when no existing data
         errorMessage = nil
 
+        // Which login the account-scoped tabs are being loaded for. Cancelling
+        // the previous task is not enough on its own: a request that already
+        // came back races the cancellation, and its results would be installed
+        // after the purge that was meant to remove them. Popular and trending
+        // are unauthenticated and deliberately not fenced.
+        let generation = accountContentGeneration
+
         do {
             switch selectedTab {
             case .popular:
@@ -1280,6 +1292,7 @@ struct InstanceBrowseView: View {
                 // Load subscriptions and feed based on instance type
                 let subscriptionChannels: [Channel]
                 let videos: [Video]
+                let hasMore: Bool
 
                 switch instance.type {
                 case .invidious:
@@ -1302,7 +1315,7 @@ struct InstanceBrowseView: View {
 
                     subscriptionChannels = enrichedSubscriptions.map { $0.toChannel(baseURL: instance.url) }
                     videos = feedResponse.videos
-                    hasMoreFeedResults = feedResponse.hasMore
+                    hasMore = feedResponse.hasMore
 
                 case .piped:
                     let api = PipedAPI(httpClient: appEnvironment.httpClient)
@@ -1316,7 +1329,7 @@ struct InstanceBrowseView: View {
                     subscriptionChannels = pipedSubscriptions.map { $0.toChannel() }
                     videos = feedVideos
                     // Piped doesn't support pagination for feed
-                    hasMoreFeedResults = false
+                    hasMore = false
 
                 default:
                     errorMessage = String(localized: "feed.error.notSupported")
@@ -1324,6 +1337,9 @@ struct InstanceBrowseView: View {
                     return
                 }
 
+                guard generation == accountContentGeneration else { return }
+
+                hasMoreFeedResults = hasMore
                 feedSubscriptions = subscriptionChannels.map { $0.enrichedThumbnail(using: appEnvironment.dataManager) }
                 feedVideos = videos
                 prefetchBranding(for: videos)
@@ -1343,6 +1359,9 @@ struct InstanceBrowseView: View {
 
                 let api = InvidiousAPI(httpClient: appEnvironment.httpClient)
                 let playlists = try await api.userPlaylists(instance: instance, sid: credential)
+
+                guard generation == accountContentGeneration else { return }
+
                 userPlaylists = playlists
             }
         } catch is CancellationError {