fix: USB/OTA update flow - version check, chunk size, UX improvements

- MainActivity: suppress toast when activity not resumed, auto-navigate to
  Settings on new firmware
- SettingsActivity: unified BLE/USB radio buttons + single update button,
  version check before OTA, show dialog before loadFirmware, spinner timeout
- BleRelayManager: expose getEspVersion()
- OtaManager: reduce chunk size 500→248 to fit BLE MTU 255
- ESP: WRITE→WRITE_NR on OTA characteristic (was silently dropping data)
- Layout/strings: remove intro text, add method radio buttons
- Bump all firmware versions to v5

src/main.cpp

@@ -34,7 +34,7 @@
 #define NVS_NAMESPACE "hmac"
 #define NVS_KEY_NAME  "key"
 #define NVS_UUID_NAME "uuid"
-#define FIRMWARE_VERSION "2"
+#define FIRMWARE_VERSION "5"
 
 NimBLECharacteristic *pCharacteristic;
 bool deviceConnected = false;
@@ -343,7 +343,7 @@ void setup() {
 
     NimBLECharacteristic *pOtaChar = pService->createCharacteristic(
         OTA_UUID,
-        NIMBLE_PROPERTY::WRITE | NIMBLE_PROPERTY::NOTIFY
+        NIMBLE_PROPERTY::WRITE_NR | NIMBLE_PROPERTY::NOTIFY
     );
     pOtaChar->setCallbacks(new OtaCharacteristicCallbacks());