Set config.yaml as active config after xmodem upload

Send $Config/Filename=config.yaml before restarting to ensure
FluidNC loads the uploaded config on boot.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flash-fluidnc.sh

@@ -332,8 +332,10 @@ try:
     else:
         print("CONFIG_UPLOAD_FAIL")
 
-    # Send restart command to apply new config
+    # Set this config as the active one and restart to apply
     time.sleep(1)
+    ser.write(b"$Config/Filename=config.yaml\n")
+    time.sleep(0.5)
     ser.write(b"$Bye\n")
     ser.close()