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/ble_service.h

@@ -0,0 +1,58 @@
+#ifndef BLE_SERVICE_H
+#define BLE_SERVICE_H
+
+#include <Arduino.h>
+
+// ============================================
+// BLE Service UUIDs (Bluetooth SIG standard)
+// ============================================
+#define HEART_RATE_SERVICE_UUID        "180D"
+#define HEART_RATE_MEASUREMENT_UUID    "2A37"
+#define BODY_SENSOR_LOCATION_UUID      "2A38"
+#define BATTERY_SERVICE_UUID           "180F"
+#define BATTERY_LEVEL_UUID             "2A19"
+#define FITNESS_MACHINE_SERVICE_UUID   "1826"
+#define TREADMILL_DATA_UUID            "2ACD"
+#define FITNESS_MACHINE_FEATURE_UUID   "2ACC"
+
+// ============================================
+// BLE Service
+// Manages BLE advertising and GATT services
+// ============================================
+class BleService {
+public:
+    static BleService& getInstance();
+
+    // Prevent copying
+    BleService(const BleService&) = delete;
+    BleService& operator=(const BleService&) = delete;
+
+    // Lifecycle
+    void setup();
+    void loop();
+
+    // Configuration
+    void setupHeartRate();
+    void setupTreadmill();
+    void stop();
+
+    // Notifications
+    void notifyHeartRate(uint8_t bpm);
+    void notifyTreadmill(uint16_t speed, int16_t incline, uint32_t distance);
+    void updateBattery(uint8_t level);
+
+    // State
+    bool isClientConnected() const { return deviceConnected; }
+
+private:
+    BleService() = default;
+
+    bool deviceConnected = false;
+    unsigned long lastNotify = 0;
+
+    void initBLE();
+};
+
+#define bleService BleService::getInstance()
+
+#endif // BLE_SERVICE_H