fix: harden resume and SSE replay

frontend/src/test/fakeEventSource.ts

@@ -1,27 +1,27 @@
 export class FakeEventSource {
   static instances: FakeEventSource[] = [];
-  onmessage: ((e: { data: string }) => void) | null = null;
+  onmessage: ((e: { data: string; lastEventId: string }) => void) | null = null;
   onopen: (() => void) | null = null;
   onerror: (() => void) | null = null;
   closed = false;
-  private listeners = new Map<string, Set<(e: { data: string }) => void>>();
+  private listeners = new Map<string, Set<(e: { data: string; lastEventId: string }) => void>>();
   constructor(public url: string) {
     FakeEventSource.instances.push(this);
   }
   /** Dispatch to the default onmessage handler (unnamed `event: message`). */
-  emit(obj: unknown) {
-    this.onmessage?.({ data: JSON.stringify(obj) });
+  emit(obj: unknown, lastEventId = "") {
+    this.onmessage?.({ data: JSON.stringify(obj), lastEventId });
   }
   /** Dispatch to handlers registered for a NAMED event (e.g. "ui_request"). */
-  emitNamed(type: string, obj: unknown) {
+  emitNamed(type: string, obj: unknown, lastEventId = "") {
     const data = JSON.stringify(obj);
-    for (const h of this.listeners.get(type) ?? []) h({ data });
+    for (const h of this.listeners.get(type) ?? []) h({ data, lastEventId });
   }
-  addEventListener(type: string, handler: (e: { data: string }) => void) {
+  addEventListener(type: string, handler: (e: { data: string; lastEventId: string }) => void) {
     if (!this.listeners.has(type)) this.listeners.set(type, new Set());
     this.listeners.get(type)!.add(handler);
   }
-  removeEventListener(type: string, handler: (e: { data: string }) => void) {
+  removeEventListener(type: string, handler: (e: { data: string; lastEventId: string }) => void) {
     this.listeners.get(type)?.delete(handler);
   }
   close() {