feat(FN-1145): harden dashboard SSE streams with replay and reconnect

- Add a shared SSE event buffer utility and wire it into planning, subtask, mission interview, and task stream routes
- Support Last-Event-ID replay semantics and robust event serialization so clients can recover missed stream events
- Add a resilient client-side reconnect wrapper and surface reconnecting state in planning, subtask breakdown, and mission interview modals
- Expand test coverage across API routes, SSE buffering behavior, reconnect handling, and mission/planning stream flows

packages/dashboard/src/subtask-breakdown.test.ts

@@ -0,0 +1,73 @@
+// @vitest-environment node
+
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  __resetSubtaskBreakdownState,
+  subtaskStreamManager,
+} from "./subtask-breakdown.js";
+
+describe("subtask-breakdown stream buffering", () => {
+  beforeEach(() => {
+    __resetSubtaskBreakdownState();
+  });
+
+  it("buffers broadcast events and forwards ids to subscribers", () => {
+    const sessionId = "subtask-session-1";
+    const callback = vi.fn();
+
+    const unsubscribe = subtaskStreamManager.subscribe(sessionId, callback);
+
+    const firstId = subtaskStreamManager.broadcast(sessionId, {
+      type: "thinking",
+      data: "delta-1",
+    });
+    const secondId = subtaskStreamManager.broadcast(sessionId, {
+      type: "subtasks",
+      data: [
+        {
+          id: "subtask-1",
+          title: "Title",
+          description: "Description",
+          suggestedSize: "S",
+          dependsOn: [],
+        },
+      ],
+    });
+
+    expect(firstId).toBe(1);
+    expect(secondId).toBe(2);
+    expect(callback).toHaveBeenNthCalledWith(1, { type: "thinking", data: "delta-1" }, 1);
+    expect(callback).toHaveBeenNthCalledWith(2, expect.objectContaining({ type: "subtasks" }), 2);
+
+    const buffered = subtaskStreamManager.getBufferedEvents(sessionId, 1);
+    expect(buffered).toHaveLength(1);
+    expect(buffered[0]).toMatchObject({ id: 2, event: "subtasks" });
+
+    unsubscribe();
+  });
+
+  it("buffers complete events without subscribers", () => {
+    const sessionId = "subtask-session-2";
+
+    const eventId = subtaskStreamManager.broadcast(sessionId, { type: "complete" });
+
+    expect(eventId).toBe(1);
+    expect(subtaskStreamManager.getBufferedEvents(sessionId, 0)).toEqual([
+      { id: 1, event: "complete", data: "{}" },
+    ]);
+  });
+
+  it("clears subscriptions and buffered events on cleanupSession", () => {
+    const sessionId = "subtask-session-3";
+    const callback = vi.fn();
+
+    subtaskStreamManager.subscribe(sessionId, callback);
+    subtaskStreamManager.broadcast(sessionId, { type: "thinking", data: "delta" });
+
+    expect(subtaskStreamManager.getBufferedEvents(sessionId, 0)).toHaveLength(1);
+
+    subtaskStreamManager.cleanupSession(sessionId);
+
+    expect(subtaskStreamManager.getBufferedEvents(sessionId, 0)).toEqual([]);
+  });
+});