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/config_service.h

@@ -0,0 +1,64 @@
+#ifndef CONFIG_SERVICE_H
+#define CONFIG_SERVICE_H
+
+#include <Arduino.h>
+#include <Preferences.h>
+
+// ============================================
+// Config Service
+// Manages persistent configuration in NVS
+// ============================================
+class ConfigService {
+public:
+    static ConfigService& getInstance();
+
+    // Prevent copying
+    ConfigService(const ConfigService&) = delete;
+    ConfigService& operator=(const ConfigService&) = delete;
+
+    // Load config from NVS
+    bool load();
+
+    // Save config to NVS
+    void save();
+
+    // Clear all config
+    void clear();
+
+    // Check if configured
+    bool isConfigured() const { return configured; }
+
+    // Getters
+    const String& getWifiSsid() const { return wifiSsid; }
+    const String& getWifiPassword() const { return wifiPassword; }
+    const String& getMqttHost() const { return mqttHost; }
+    uint16_t getMqttPort() const { return mqttPort; }
+    const String& getDeviceId() const { return deviceId; }
+
+    // Setters
+    void setWifiCredentials(const String& ssid, const String& password);
+    void setMqttConfig(const String& host, uint16_t port);
+    void setDeviceId(const String& id);
+
+    // Get unique AP name based on chip ID
+    String getAPName() const;
+
+    // Get unique default device ID based on chip ID
+    String getDefaultDeviceId() const;
+
+private:
+    ConfigService() = default;
+
+    bool configured = false;
+    String wifiSsid = "";
+    String wifiPassword = "";
+    String mqttHost = "";
+    uint16_t mqttPort = 1883;
+    String deviceId = "";
+
+    Preferences preferences;
+};
+
+#define configService ConfigService::getInstance()
+
+#endif // CONFIG_SERVICE_H