Calibration countdown : user can stop pressing

A short red wheel is displayed after the LED countdown has been
completed. It prevents a new countdown from starting, or night mode to
be activated.

Since a red wheel is displayed at the start of the calibration process
anyway (until measurements are stable enough), it should hopefully not
be too surprising.

ampel-firmware/ampel-firmware.ino

@@ -190,7 +190,9 @@ void checkFlashButton() {
     } else {
       Serial.println(F("Flash has been pressed for a long time. Keep it pressed for calibration."));
       if (led_effects::countdownToZero()) {
+        Serial.println(F("You can now release the button."));
         sensor::startCalibrationProcess();
+        led_effects::showKITTWheel(color::red, 2);
       }
     }
     led_effects::onBoardLEDOff();