Switch the motors off when the designs are done or stopped

After the last design (or a Stop), the player sends $SLP (GRBL and FluidNC
sleep), so the motors stop holding and the table is quiet. The page shows
"Motors off"; the next design wakes the controller (reset + unlock) and homes
first, since the arm can be moved by hand while the motors are off. The jog
panel wakes it too. Setting: motion.motors_off_when_done (default true).

Checked on the real Uno: Idle -> $SLP -> Sleep -> reset -> Alarm -> $X -> Idle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

BUILD.md

@@ -254,6 +254,7 @@ python3 -m sandtable web
 This opens a page in your browser with the design library:
 - click a design to draw it; it homes and erases the sand first;
 - **Play all** runs the whole list, shuffled and repeating;
+- when the designs finish (or you press Stop), the motors switch off and the page shows "Motors off". The next design homes first;
 - **Upload** takes `.thr` files (from [sandify.org](https://sandify.org)) and `.svg` line drawings.
 
 Leave the terminal open while the table runs. See [`software/README.md`](software/README.md) for everything else.