Fix MQTT connection loss and display freeze after extended uptime

[Copilot]

After weeks of uptime, the MQTT connection would permanently die and display refreshes would become extremely sluggish, requiring a reboot to recover. Root cause: multiple blocking operations in the reconnect path were starving loop().

Bugs fixed

networking.cpp

• Double mqttClient.connect() call — the first call's result was silently discarded, causing two full blocking TCP/TLS handshakes on every reconnect attempt
• delay(1000) in reconnectWiFi() — blocked the entire loop() for 1 second every 5 seconds whenever WiFi was lost; removed since WiFi.setAutoReconnect(true) was already set
• No timeout on WiFiClientSecure — a failed MQTT connect could block for 10–30 s per attempt; capped at 5 s via espClient.setTimeout(5)
• lastReconnectTry uninitialized — added explicit = 0 in constructor initializer list

// Before: two blocking connect attempts, result of first discarded
mqttClient.disconnect();
mqttClient.connect(clientId.c_str(), MQTT_USERNAME, MQTT_PASSWORD); // ignored
bool connected = mqttClient.connect(clientId.c_str(), MQTT_USERNAME, MQTT_PASSWORD);

// After: single attempt with bounded timeout
mqttClient.disconnect();
bool connected = mqttClient.connect(clientId.c_str(), MQTT_USERNAME, MQTT_PASSWORD);

display_handler.cpp

• uint16_t underflow on alarm ring-buffer index — writeIndex - scrollPosition wraps to ~65534 when scrollPosition > writeIndex, causing an out-of-bounds read on alarmHistory[]; fixed to (writeIndex - scrollPosition + MAX_ALARMS) % MAX_ALARMS