v1.2.0: Firmware refactor to service-oriented architecture + WiFi fixes

Firmware:
- Fixed WiFi reconnection bug after power cycle by adding WiFi.persistent(false)
  and disabling auto-connect/auto-reconnect
- Fixed AP mode not broadcasting by properly handling WIFI_AP_STA mode transitions
- Refactored to service-oriented architecture:
  - device_state: Central state management with event callbacks
  - config_service: NVS persistence (renamed from config_manager)
  - wifi_service: WiFi STA/AP management (extracted from main.cpp)
  - mqtt_service: MQTT client and message routing (extracted from main.cpp)
  - ble_service: BLE GATT services (renamed from ble_services)
  - web_service: HTTP configuration portal (renamed from web_portal)
- main.cpp reduced from ~470 lines to ~60 lines (thin orchestrator)
- Each service is self-contained with setup()/loop() pattern

Backend:
- Added heart rate variation endpoint
- UI improvements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

firmware/esp32_ble_sim/include/services/mqtt_service.h

@@ -0,0 +1,45 @@
+#ifndef MQTT_SERVICE_H
+#define MQTT_SERVICE_H
+
+#include <Arduino.h>
+
+// ============================================
+// MQTT Service
+// Manages MQTT connection and message routing
+// ============================================
+class MqttService {
+public:
+    static MqttService& getInstance();
+
+    // Prevent copying
+    MqttService(const MqttService&) = delete;
+    MqttService& operator=(const MqttService&) = delete;
+
+    // Lifecycle
+    void setup();
+    void loop();
+
+    // State
+    bool isConnected() const { return mqttConnected; }
+
+    // Publishing
+    void publishStatus();
+    void publishValues();
+
+private:
+    MqttService() = default;
+
+    bool mqttConnected = false;
+    unsigned long lastMqttAttempt = 0;
+    unsigned long lastStatusReport = 0;
+
+    void connectToMQTT();
+    void handleMessage(char* topic, byte* payload, unsigned int length);
+
+    // Static callback wrapper for PubSubClient
+    static void messageCallback(char* topic, byte* payload, unsigned int length);
+};
+
+#define mqttService MqttService::getInstance()
+
+#endif // MQTT_SERVICE_H