Optimize WebUI streaming and long history rendering

Batch stream deltas, window long transcripts, lazy-load syntax highlighting, and refine activity/composer interactions.

Add title refresh retries plus tests for streaming, windowing, code blocks, and live activity behavior.

webui/src/hooks/useDeferredTitleRefresh.ts

@@ -0,0 +1,68 @@
+import { useCallback, useEffect, useRef } from "react";
+
+import type { ChatSummary } from "@/lib/types";
+
+const TITLE_REFRESH_RETRY_DELAYS_MS = [1_000, 3_000, 7_000] as const;
+
+function hasGeneratedTitle(session: ChatSummary | null): boolean {
+  return !!session?.title?.trim();
+}
+
+/**
+ * The server generates WebUI titles after the main turn has already ended.
+ * Refresh once immediately, then retry lightly for untitled sessions so the
+ * async title appears even if the websocket metadata notification is delayed.
+ */
+export function useDeferredTitleRefresh(
+  activeSession: ChatSummary | null,
+  refresh: () => Promise<void>,
+  retryDelaysMs: readonly number[] = TITLE_REFRESH_RETRY_DELAYS_MS,
+): () => void {
+  const activeSessionRef = useRef(activeSession);
+  const timersRef = useRef<ReturnType<typeof setTimeout>[]>([]);
+  activeSessionRef.current = activeSession;
+
+  const clearTimers = useCallback(() => {
+    for (const timer of timersRef.current) {
+      clearTimeout(timer);
+    }
+    timersRef.current = [];
+  }, []);
+
+  useEffect(() => clearTimers, [clearTimers]);
+
+  useEffect(() => {
+    clearTimers();
+  }, [activeSession?.key, clearTimers]);
+
+  useEffect(() => {
+    if (hasGeneratedTitle(activeSession)) {
+      clearTimers();
+    }
+  }, [activeSession, clearTimers]);
+
+  return useCallback(() => {
+    void refresh();
+
+    const sessionAtTurnEnd = activeSessionRef.current;
+    if (!sessionAtTurnEnd || hasGeneratedTitle(sessionAtTurnEnd)) {
+      return;
+    }
+
+    clearTimers();
+    for (const delayMs of retryDelaysMs) {
+      const timer = setTimeout(() => {
+        const latest = activeSessionRef.current;
+        if (
+          !latest ||
+          latest.key !== sessionAtTurnEnd.key ||
+          hasGeneratedTitle(latest)
+        ) {
+          return;
+        }
+        void refresh();
+      }, delayMs);
+      timersRef.current.push(timer);
+    }
+  }, [clearTimers, refresh, retryDelaysMs]);
+}