fix(chat): hide the load-older button once history is confirmed complete

The button's visibility mixed the sync meta with the prefetch-cache hint;
a stale prefetch entry (cursor recorded at the initial page) could keep
the affordance alive after the user had already loaded to the top. Sync
now exposes an explicit isComplete (positive confirmation from a fetch,
distinct from !hasMore on unpopulated meta) and it overrides the prefetch
hint, which stays in effect only before the meta knows anything.

packages/ui/src/sync/use-sync.ts

@@ -555,6 +555,14 @@ export function useSync() {
     [getMetaFor],
   )
 
+  // True only when a fetch has positively confirmed the history is fully
+  // loaded (no next cursor). Distinct from !hasMore(), which is also true for
+  // sessions whose meta simply hasn't been populated yet.
+  const isComplete = useCallback(
+    (sessionID: string) => getMetaFor(sessionID).complete,
+    [getMetaFor],
+  )
+
   // Optimistic add (for prompt submission)
   const optimisticAdd = useCallback(
     (input: { sessionID: string; directory?: string | null; message: Message; parts: Part[] }) => {
@@ -610,11 +618,12 @@ export function useSync() {
       loadMore,
       hasMore,
       isLoading,
+      isComplete,
       optimistic: {
         add: optimisticAdd,
         remove: optimisticRemove,
       },
     }),
-    [syncSession, loadMore, hasMore, isLoading, optimisticAdd, optimisticRemove],
+    [syncSession, loadMore, hasMore, isLoading, isComplete, optimisticAdd, optimisticRemove],
   )
 }