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/sandtable/grbl.py

@@ -346,7 +346,33 @@ class Controller:
             self._presses = 0
         return self.query()
 
+    def motors_off(self):
+        """Switch the motors off with the sleep command ($SLP; FluidNC has it too). They stop
+        holding, so the arm turns by hand and the position is lost: before moving again the
+        controller needs waking (a reset, see wake_up) and the table homing."""
+        self.guard = None
+        if self.query().state != 'Sleep':
+            self.command('$SLP')
+        self.query()
+
+    def wake_up(self):
+        """Out of sleep: only a reset wakes it, and it then waits in ALARM until unlocked."""
+        if self.query().state != 'Sleep':
+            return
+        self.realtime(RT_RESET)
+        t_end, seen = time.monotonic() + 3, None
+        while time.monotonic() < t_end:
+            for line in self._lines():
+                if 'grbl' in line.lower() or 'fluidnc' in line.lower():
+                    seen = time.monotonic()
+            if seen and time.monotonic() - seen > 0.2:
+                break
+            time.sleep(0.02)
+        self.pending.clear()
+        self.query()
+
     def unlock(self):
+        self.wake_up()
         if self.query().state == 'Alarm':
             self.command('$X')