Read a playlist back before writing over it

Ten review findings in the remote-playlist work, all of them the same
shape: something acted on what it remembered instead of what the server
says now.

- A confirmed-gone playlist is forgotten whole — snapshot, list entry,
  and both ordering fences — so a refresh that started before the delete
  can no longer republish it, and a list refetch that fails offline no
  longer leaves it on screen.
- `.accountsDidChange` discards account-scoped content when the account
  it belonged to is no longer the effective one, as the status
  notification already did.
- A tab that stops being offered no longer stays selected: the browse
  view moves to the first available one, which reloads by itself.
- A Handoff account id is accepted only when that account is on the
  instance the playlist lives on.
- Composite writes moved up into `AccountService`, over a transport whose
  every method is exactly one request: create-with-privacy, rename, and
  set-description. The credential is rechecked between each pair, so a
  sign-out between the two halves of a create or an edit can no longer
  send the second half under the previous user's secret.
- Editing a playlist no longer forces it private. Invidious serialises
  visibility as `isListed` — `privacy.public?`, so unlisted and private
  are indistinguishable — which the snapshot now carries; the picker
  starts there, and sends nothing at all unless it was moved. Invidious
  keeps the stored visibility when the PATCH omits it (measured), so an
  untouched picker leaves it exactly as it was.
- Tapping Edit on a server-side playlist awaits an authenticated refetch
  first and populates the form from it; a failed refetch shows an error
  and no sheet.
- Playlist rows are keyed by row rather than by video id, which two rows
  of the same video share.
- The add-to-playlist sheet offers only accounts whose instance is
  enabled and that hold a credential, alongside the write capability.

Yattee/Services/RemotePlaylistsCache.swift

@@ -158,7 +158,7 @@ final class RemotePlaylistsCache {
     func deletePlaylist(id: String, for account: Account) async throws {
         do {
             try await accountService.deletePlaylist(id: id, for: account)
-            forgetSnapshot(id: id, for: account.id)
+            forgetPlaylist(id: id, for: account.id)
             await reconcileList(for: account)
         } catch {
             // The delete may have landed even though the answer did not, so the
@@ -347,8 +347,7 @@ final class RemotePlaylistsCache {
         do {
             try await fetchSnapshot(id: id, for: account)
         } catch let error as APIError where Self.saysPlaylistIsGone(error) {
-            forgetSnapshot(id: id, for: account.id)
-            forgetListEntry(id: id, for: account.id)
+            forgetPlaylist(id: id, for: account.id)
         } catch {
             // Swallowed: see the doc comment on `reconcileList(for:)`.
         }
@@ -365,17 +364,22 @@ final class RemotePlaylistsCache {
         }
     }
 
-    private func forgetSnapshot(id: String, for accountID: UUID) {
+    /// Drops a playlist the server is known no longer to have.
+    ///
+    /// Three things, and each one is load-bearing. The snapshot goes, so a
+    /// detail view still open on it has nothing left to show. The list entry
+    /// goes, because the list is refetched right after but that refetch is
+    /// allowed to fail — offline, a playlist known to be gone would otherwise
+    /// sit in the list until the next successful fetch. And both fences move
+    /// past every request already in flight: a refresh that started before the
+    /// delete describes a playlist that still existed, and arriving late does
+    /// not make it exist again.
+    private func forgetPlaylist(id: String, for accountID: UUID) {
+        appliedSnapshotSequence[SnapshotKey(accountID: accountID, playlistID: id)] = nextSequence()
+        appliedListSequence[accountID] = nextSequence()
+
         snapshots[accountID]?[id] = nil
         snapshotFetchDates[accountID]?[id] = nil
-    }
-
-    /// Takes a playlist out of the account's last-fetched list.
-    ///
-    /// The list is refetched right after, which normally says the same thing —
-    /// but a refetch that cannot reach the server would otherwise leave a
-    /// playlist that is known to be gone sitting in it.
-    private func forgetListEntry(id: String, for accountID: UUID) {
         lists[accountID]?.removeAll { $0.id.playlistID == id }
     }