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>

software/tests/test_sandtable.py

@@ -241,6 +241,8 @@ class TestCommands(unittest.TestCase):
         self.assertTrue(any(l.startswith('$J=') for l in f.lines))          # homed first
         self.assertGreater(sum(1 for l in f.lines if l.startswith('G1X')), 100)
         self.assertLessEqual(f.max_rx, 128)
+        self.assertEqual(f.lines[-1], '$SLP')                               # motors off at the end
+        self.assertTrue(f.sleeping)
 
 
 if __name__ == '__main__':