Improve MQTT resilience, clarify behavior, and minor UX

Switch to Paho v2 callbacks; add loop_start() and reconnect_delay_set() for auto-reconnect
Rework on_connect/on_disconnect to v2 signatures; handle session_present and reconnection flows
Gate heartbeats with client.is_connected() and add short retry on rc=4 (NO_CONN)
Re-send discovery after reconnect; ensure re-subscription to all topics
Add startup terminal message with ISO timestamp in simclient.py
Docs: update README and Copilot instructions with reconnection/heartbeat guidance and benign rc=4 notes

.github/copilot-instructions.md

@@ -33,6 +33,11 @@ The server now performs all PPTX → PDF conversion. The client only ever downlo
 ### MQTT Communication Patterns
 - **Discovery**: `infoscreen/discovery` → `infoscreen/{client_id}/discovery_ack`
 - **Heartbeat**: Regular `infoscreen/{client_id}/heartbeat` messages
+### MQTT Reconnection & Heartbeat (Nov 2025)
+- The client uses Paho MQTT v2 callback API with `client.loop_start()` and `client.reconnect_delay_set()` to handle automatic reconnection.
+- `on_connect` re-subscribes to all topics (`discovery_ack`, `config`, `group_id`, current group events) and re-sends discovery on reconnect to re-register with the server.
+- Heartbeats are gated by `client.is_connected()` and retry once on `NO_CONN` (rc=4). Occasional rc=4 warnings are normal right after broker restarts or brief network stalls and typically followed by a successful heartbeat.
+- Do not treat single rc=4 heartbeat warnings as failures. Investigate only if multiple consecutive heartbeats fail without recovery.
 - **Dashboard**: Screenshot transmission via `infoscreen/{client_id}/dashboard`
 - **Group Assignment**: Server sends group via `infoscreen/{client_id}/group_id`
 - **Events**: Content commands via `infoscreen/events/{group_id}`