fix(server): prevent streaming hang during long agent sessions (#1088)

* fix(server): increase WS buffer/replay limits and add backpressure warning

During long-running agent sessions (e.g. ultrawork loops with many tool
calls), the browser WebSocket client can briefly fall behind the server.
When the outbound buffer exceeds the limit, the server force-disconnects
with close code 1013, and the small replay buffer (512 events) is
insufficient to recover all missed events — leaving the UI permanently
stalled.

Changes:
- Raise MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES from 4 MB to 16 MB to
  tolerate larger bursts without disconnecting
- Add MESSAGE_STREAM_WS_BACKPRESSURE_WARN_BYTES (12 MB) threshold that
  sends a one-shot "backpressure" frame to the client before the hard
  disconnect, giving it a chance to shed low-priority updates
- Raise MESSAGE_STREAM_GLOBAL_REPLAY_LIMIT from 512 to 2048 so more
  events survive brief reconnection gaps
- Add tests for the backpressure warning behavior (emit, dedup, reset)

* fix(ui): batch event flushes under backpressure

---------
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/web/server/lib/event-stream/global-hub.js

@@ -1,6 +1,8 @@
 import { createUpstreamSseReader } from './upstream-reader.js';
 
-export const MESSAGE_STREAM_GLOBAL_REPLAY_LIMIT = 512;
+// Raised from 512 → 2048 to improve recovery after brief disconnects during
+// long-running agent sessions where many events accumulate quickly.
+export const MESSAGE_STREAM_GLOBAL_REPLAY_LIMIT = 2048;
 
 export function createGlobalMessageStreamHub({
   buildOpenCodeUrl,