refactor: drop firmware-version display, move Enable toggle to Controller

Firmware version is already printed to the log at connect() time
(HendiCtrl/Pololu1376) - remove the live GUI/websocket firmware
reporting entirely (Connectable, HeaterHendi/StirrerPololu1376,
Heater/StirrerTask, web GUI) as redundant.

Also move the Heater panel's Closed-loop checkbox into the Controller
panel and rename it "Enable" - the Controller panel now shows a
live Enabled (green) / Disabled (gray) status badge next to its
heading, matching the Connected/Disconnected badges on the Heater/
Stirrer panels.

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

components/connectable.py

@@ -4,16 +4,15 @@ from utils.value import AttributeChange
 class Connectable(AttributeChange):
 	"""Observable hardware-connection state shared by AHeater/AStirrer.
 
-	Simulated devices are always connected and report no firmware version;
-	real (serial) devices start disconnected and only flip once connect()
-	actually reaches the hardware.
+	Simulated devices are always connected; real (serial) devices start
+	disconnected and only flip once connect() actually reaches the
+	hardware.
 	"""
 
 	def __init__(self, simulated):
 		AttributeChange.__init__(self)
 		self.simulated = simulated
 		self.connected = simulated
-		self.firmware_version = None
 
 	def connect(self):
 		return self.connected