fix: harden resume and SSE replay

frontend/src/stream/useSessionStream.ts

@@ -1,4 +1,4 @@
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { BASE } from "../api/client";
 import { joinBackendPath } from "../api/runtime-config";
 import { useSessionStore } from "../store/sessionStore";
@@ -7,15 +7,24 @@ import type { StreamEvent } from "../api/types";
 export function useSessionStream(sessionId: string | null, generation = 0) {
   const [connected, setConnected] = useState(false);
   const applyEvent = useSessionStore((s) => s.applyEvent);
+  const cursor = useRef({ sessionId: null as string | null, lastEventId: "" });
+
+  if (cursor.current.sessionId !== sessionId) {
+    cursor.current = { sessionId, lastEventId: "" };
+  }
 
   useEffect(() => {
     if (!sessionId) return;
 
-    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events`));
+    const query = cursor.current.lastEventId
+      ? `?lastEventId=${encodeURIComponent(cursor.current.lastEventId)}`
+      : "";
+    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events${query}`));
     es.onopen = () => setConnected(true);
     es.onerror = () => setConnected(false);
 
     const handle = (ev: MessageEvent) => {
+      if (ev.lastEventId) cursor.current.lastEventId = ev.lastEventId;
       try {
         applyEvent(JSON.parse(ev.data) as StreamEvent);
       } catch {