Auto-detect upload port via ID? serial query (issue #11)

- Add ID? command to firmware: server responds 'SylDa_Server', client 'SylDa_Client'
- Add tools/pio_upload_port.py POST-script that queries connected ESP32s
- Remove hardcoded upload_port from platformio.ini
- Script logic: 1 ESP32 → use directly, 2+ → ID? query, no match → prompt user
- Requires pyserial for serial queries (graceful fallback if missing)

src/client.cpp

@@ -263,6 +263,8 @@ void loop() {
                 } else {
                     Serial.println("NO_KEY");
                 }
+            } else if (serialBuf == "ID?") {
+                Serial.println("SylDa_Client");
             } else if (serialBuf == "VERSION?") {
                 Serial.print("BUILD_NR:");
                 Serial.println(FIRMWARE_VERSION_CLIENT);