Replace Serial.* logging with ESP_LOG* macros (issue #8)

- Convert all Serial.println/printf debug logging to ESP_LOGE/W/I/D
- Add TAG-based log identification (server/client)
- Lower CORE_DEBUG_LEVEL from 5 to 3 for production
- Keep Serial.printf for USB serial protocol commands (KEY?, VERSION?, etc.)

platformio.ini

@@ -5,7 +5,7 @@ framework = arduino
 board_build.arduino.memory_type = qio_qspi
 build_flags =
     -DESP32S3
-    -DCORE_DEBUG_LEVEL=5
+    -DCORE_DEBUG_LEVEL=3
     -DARDUINO_USB_CDC_ON_BOOT=1
     -DARDUINO_USB_MODE=1
 lib_deps =
@@ -24,7 +24,7 @@ framework = arduino
 board_build.arduino.memory_type = qio_qspi
 build_flags =
     -DESP32S3
-    -DCORE_DEBUG_LEVEL=5
+    -DCORE_DEBUG_LEVEL=3
     -DARDUINO_USB_CDC_ON_BOOT=1
     -DARDUINO_USB_MODE=1
     -DDEVICE_ID=1