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/web_service.h

@@ -0,0 +1,35 @@
+#ifndef WEB_SERVICE_H
+#define WEB_SERVICE_H
+
+#include <Arduino.h>
+
+// ============================================
+// Web Service
+// Manages HTTP server for configuration portal
+// ============================================
+class WebService {
+public:
+    static WebService& getInstance();
+
+    // Prevent copying
+    WebService(const WebService&) = delete;
+    WebService& operator=(const WebService&) = delete;
+
+    // Lifecycle
+    void setup();
+    void loop();
+
+private:
+    WebService() = default;
+
+    void handleRoot();
+    void handleGetStatus();
+    void handlePostConfig();
+    void handleReset();
+    void handleResetDistance();
+    void handleSetBattery();
+};
+
+#define webService WebService::getInstance()
+
+#endif // WEB_SERVICE_H