refactor: message list optimizations and single messageLimit setting (#409)

* feat: message list refactoring and optimizations

* refactor: collapse 3 memory-limit settings into single messageLimit

- Replace memoryLimitHistorical/Viewport/ActiveSession with one `messageLimit` (default 200)
- Background trim derived automatically (limit * 0.6), not user-facing
- Fix stale historyLimit overriding current messageLimit on session reload
- Remove streaming cap that reduced fetch to VIEWPORT_MESSAGES during active streams
- Deduplicate detectTurns() call in TurnGroupingContext (reuse staticValue.turns)
- Zustand v3 migration: collapse old fields, preserve user-customized values
- Server: sanitize single messageLimit field (10-500)

packages/ui/src/components/chat/MessageList.tsx

@@ -25,6 +25,8 @@ interface MessageListProps {
     hasMoreAbove: boolean;
     isLoadingOlder: boolean;
     onLoadOlder: () => void;
+    hasRenderEarlier?: boolean;
+    onRenderEarlier?: () => void;
     scrollToBottom?: (options?: { instant?: boolean; force?: boolean }) => void;
     scrollRef?: React.RefObject<HTMLDivElement | null>;
 }
@@ -198,6 +200,8 @@ const MessageList: React.FC<MessageListProps> = ({
     hasMoreAbove,
     isLoadingOlder,
     onLoadOlder,
+    hasRenderEarlier,
+    onRenderEarlier,
     scrollToBottom,
 }) => {
     React.useEffect(() => {
@@ -256,6 +260,18 @@ const MessageList: React.FC<MessageListProps> = ({
     return (
         <TurnGroupingProvider messages={displayMessages}>
             <div>
+                {hasRenderEarlier && (
+                    <div className="flex justify-center py-3">
+                        <button
+                            type="button"
+                            onClick={onRenderEarlier}
+                            className="text-xs uppercase tracking-wide text-muted-foreground/80 hover:text-foreground"
+                        >
+                            Render earlier messages
+                        </button>
+                    </div>
+                )}
+
                 {hasMoreAbove && (
                     <div className="flex justify-center py-3">
                         {isLoadingOlder ? (