Stop the legacy validator from overruling a local selection

Local data is always a valid context. The legacy validator predates
that: it treats local as unavailable without a Yattee Server, which is a
feed concern the feed reports itself — and since the correction now
moves the data context with it, that would have quietly put users on
somebody's backend account, overwriting a local choice made on another
device.

Stale feed responses no longer touch anything. Rejecting only the videos
still let a request from a login the cache had moved off overwrite the
new one's loading state, error state and pagination — replacement
requests are allowed to overlap, so both were in flight.

The unreachable retry timers are cancelled when the namespace reloads. A
migrated account reuses its instance's UUID, so the same ID can exist
under the next Apple account, and an old timer would declare that
account reachable on the strength of a verdict about someone else's.

Rewriting a credential's sync flag is skipped when it already matches
and restores the original if the re-add fails. Changing that flag means
delete-then-add, and the delete propagates to every device — turning
sync on should never be able to log the user out.

Yattee/Services/Subscriptions/SubscriptionAccountValidator.swift

@@ -127,6 +127,19 @@ final class SubscriptionAccountValidator {
             return
         }
 
+        // Local is always a valid context. This validator predates that: it
+        // treats local as unavailable unless a Yattee Server is configured,
+        // which is a *feed* concern — the feed says so itself — not a reason to
+        // move the user onto somebody's backend account, or to overwrite a
+        // local choice synced from another device.
+        if accountsManager?.currentContext.isLocal == true {
+            LoggingService.shared.debug(
+                "Leaving the local selection alone — local data is always available",
+                category: .general
+            )
+            return
+        }
+
         // A selection whose account still exists is never corrected here. This
         // validator predates account-scoped credentials and reads "not signed
         // in" as "not available", which is now routinely temporary — an