feat: add connect/disconnect status for real heater/stirrer hardware

Heater and Stirrer can be real serial hardware (hendi, Pololu1376), but
there was no connection concept at all - the constructors opened the
port and crashed the whole server if the device was missing, with no
way to see connection status or firmware version and no way to
reconnect without a restart.

Adds an observable Connectable mixin (components/connectable.py) shared
by AHeater/AStirrer; real devices defer opening the serial port to an
explicit connect(), auto-connect on server startup, and surface
Connected/FirmwareVersion/Simulated plus manual Connect/Disconnect over
the web GUI. Heating/stirring and Sud Start are all gated on connection
state, and a disconnect mid-brew force-stops the run via the same path
as a manual Stop.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YaPLuRPpyjWcwhMvCvpHCL

components/actor/pololu1376.py

@@ -50,17 +50,50 @@ class Varid(enum.Enum):
 
 class Pololu1376:
 	def __init__(self, serial_port):
+		# Deferred-open pyserial idiom: serial_port is constructed with no
+		# port set (see components/actor/stirrerpololu1376.py), so it isn't
+		# actually opened until connect() is called - a missing/unplugged
+		# device then doesn't crash the whole server at construction time.
 		self.ser = serial_port
-		self.ser.set_output_flow_control(False)
+		self.firmware_version = None
+
+	def is_connected(self):
+		return self.ser.is_open
+
+	def connect(self):
+		if self.ser.is_open:
+			return True
 		try:
 			self.ser.open()
 		except serial.SerialException:
+			try:
+				self.ser.close()
+				self.ser.open()
+			except serial.SerialException as e:
+				print(f"Pololu1376: connect failed: {e}")
+				return False
+		# set_output_flow_control() requires the port to already be open -
+		# can only happen here, after open() above, not at construction time.
+		self.ser.set_output_flow_control(False)
+		try:
+			self.stop()
+			self.firmware_version = self.get_firmware_version()
+			print("Pololu1376 F/W-Version:", self.firmware_version)
+			return True
+		except Exception as e:
+			print(f"Pololu1376: connect failed: {e}")
 			self.ser.close()
-			self.ser.open()
+			self.firmware_version = None
+			return False
 
-		self.stop()
-
-		print("Pololu1376 F/W-Version:", self.get_firmware_version())
+	def disconnect(self):
+		if self.ser.is_open:
+			try:
+				self.stop()
+			except Exception:
+				pass
+			self.ser.close()
+		self.firmware_version = None
 
 	def send(self, data):
 		self.ser.write((data + '\r\n').encode())
@@ -126,9 +159,12 @@ class Pololu1376:
 
 
 if __name__ == '__main__':
-	ser = serial.Serial("/dev/ttyACM0", 115200)
+	ser = serial.Serial()
+	ser.port = "/dev/ttyACM0"
+	ser.baudrate = 115200
 
 	drv = Pololu1376(serial_port=ser)
+	drv.connect()
 	drv.go()
 	time.sleep(1)