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/src/services/config_service.cpp

@@ -0,0 +1,85 @@
+#include "services/config_service.h"
+#include "config.h"
+
+ConfigService& ConfigService::getInstance() {
+    static ConfigService instance;
+    return instance;
+}
+
+bool ConfigService::load() {
+    preferences.begin(NVS_NAMESPACE, true);  // Read-only
+
+    configured = preferences.getBool("configured", false);
+    wifiSsid = preferences.getString("wifi_ssid", "");
+    wifiPassword = preferences.getString("wifi_pass", "");
+    mqttHost = preferences.getString("mqtt_host", "");
+    mqttPort = preferences.getUShort("mqtt_port", DEFAULT_MQTT_PORT);
+    deviceId = preferences.getString("device_id", getDefaultDeviceId());
+
+    preferences.end();
+
+    return configured;
+}
+
+void ConfigService::save() {
+    preferences.begin(NVS_NAMESPACE, false);  // Read-write
+
+    preferences.putBool("configured", configured);
+    preferences.putString("wifi_ssid", wifiSsid);
+    preferences.putString("wifi_pass", wifiPassword);
+    preferences.putString("mqtt_host", mqttHost);
+    preferences.putUShort("mqtt_port", mqttPort);
+    preferences.putString("device_id", deviceId);
+
+    preferences.end();
+
+    Serial.println("Configuration saved to NVS");
+}
+
+void ConfigService::clear() {
+    preferences.begin(NVS_NAMESPACE, false);
+    preferences.clear();
+    preferences.end();
+
+    configured = false;
+    wifiSsid = "";
+    wifiPassword = "";
+    mqttHost = "";
+    mqttPort = DEFAULT_MQTT_PORT;
+    deviceId = "";
+
+    Serial.println("Configuration cleared");
+}
+
+void ConfigService::setWifiCredentials(const String& ssid, const String& password) {
+    wifiSsid = ssid;
+    wifiPassword = password;
+    if (ssid.length() > 0) {
+        configured = true;
+    }
+}
+
+void ConfigService::setMqttConfig(const String& host, uint16_t port) {
+    mqttHost = host;
+    mqttPort = port;
+}
+
+void ConfigService::setDeviceId(const String& id) {
+    deviceId = id.length() > 0 ? id : getDefaultDeviceId();
+}
+
+String ConfigService::getAPName() const {
+    uint32_t chipId = 0;
+    for (int i = 0; i < 17; i += 8) {
+        chipId |= ((ESP.getEfuseMac() >> (40 - i)) & 0xff) << i;
+    }
+    return String(AP_SSID_PREFIX) + String(chipId, HEX);
+}
+
+String ConfigService::getDefaultDeviceId() const {
+    uint32_t chipId = 0;
+    for (int i = 0; i < 17; i += 8) {
+        chipId |= ((ESP.getEfuseMac() >> (40 - i)) & 0xff) << i;
+    }
+    return String(DEFAULT_DEVICE_ID_PREFIX) + String(chipId, HEX);
+}