Refuse to repair a context that is not this identity's

Foreground activation repairs a selection whose account this device
knows was deleted. The check for whether that is safe was the identity
scope being resolved, which it already is in the window between an Apple
account change moving the namespace and the reload refilling memory —
so the repair weighed the previous identity's selection against the
previous identity's tombstones and announced the result to a user who
had not been shown an account yet. Fenced on the loaded namespace, the
same way saves are.

Three smaller things around the same theme:

- The destination rows forgot their membership answers on a credential
  or account change but not on an instance change, which adds a row to
  the picker while it is open. Unknown reads as unsubscribed, so that
  row offered to subscribe a channel the account already has.

- The subscriptions settings count applied whatever the fetch returned.
  A selection change is fenced by the service's provider generation, but
  the fallback to local is driven by availability and announces nothing,
  so the context is captured before the fetch and rechecked after it.

- Deleting a source from the browser runs the same cascade as deleting
  it from Settings, and was the one dialog left that did not say the
  logins go with it.

Yattee/Views/Settings/SubscriptionsSettingsView.swift

@@ -353,14 +353,23 @@ struct SubscriptionsSettingsView: View {
     }
 
     /// Refreshes the account-scoped subscription count shown in the export footer.
+    ///
+    /// The context the fetch was started for is rechecked before its answer is
+    /// applied. A selection change would already have been fenced by the
+    /// service's provider generation, but the fallback to local is driven by the
+    /// account's availability and announces nothing — so a count still in flight
+    /// would otherwise land under the local heading it never came from.
     private func refreshAccountSubscriptionCount() async {
         guard let subscriptionService else { return }
 
         if let cached = subscriptionService.cachedSubscriptionCount {
             accountSubscriptionCount = cached
         }
-        if !effectiveContext.isLocal {
+
+        let context = effectiveContext
+        if !context.isLocal {
             if let channels = try? await subscriptionService.fetchSubscriptions() {
+                guard effectiveContext == context else { return }
                 accountSubscriptionCount = channels.count
             }
         }