fix(web-ui): harden SSE broadcaster — drop-oldest on full queue, exit on no subscribers, reattach reconnect handlers

- _broadcast: on queue.Full drop the oldest item and retry the put
  instead of removing the client from _clients — a slow tab now stays
  subscribed and receives the latest data rather than being silently
  ejected
- _broadcast: break instead of continue when _clients is empty so the
  background generator thread exits rather than spinning indefinitely;
  subscribe() already restarts it on the next connection
- base.html: expose _statsOpenHandler, _statsErrorHandler, and
  _displayErrorHandler as window properties so reconnectSSE() can
  reattach them after replacing the EventSource instances
- app.js: reconnectSSE() now reattaches those handlers after creating
  each new EventSource so the status badge and display-stream console
  logging survive a manual reconnect

Heartbeat path (~line 646) is a queue read (q.get), not a write; no
queue.Full can occur there so no change needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/app.py

@@ -445,14 +445,23 @@ class _StreamBroadcaster:
         for data in self._generator_factory():
             with self._lock:
                 if not self._clients:
-                    continue
-                dead = set()
+                    # No subscribers — exit so the thread doesn't spin indefinitely.
+                    # subscribe() will restart it when a new client arrives.
+                    break
                 for q in self._clients:
                     try:
                         q.put_nowait(data)
                     except queue.Full:
-                        dead.add(q)
-                self._clients -= dead
+                        # Client is reading too slowly; drop the oldest item and
+                        # deliver the latest so the queue never stalls the client.
+                        try:
+                            q.get_nowait()
+                        except queue.Empty:
+                            pass
+                        try:
+                            q.put_nowait(data)
+                        except queue.Full:
+                            pass
 
 # System status generator for SSE
 def system_status_generator():