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

web/index.html

@@ -51,6 +51,12 @@
 		</div>
 		<div class="panel">
 			<h3>Heater</h3>
+			<div class="device-status-row">
+				<span id="heater-status" class="status-disconnected">Disconnected</span>
+				<span id="heater-firmware" class="device-firmware"></span>
+				<button id="btn-heater-connect">Connect</button>
+				<button id="btn-heater-disconnect" disabled>Disconnect</button>
+			</div>
 			<label class="checkbox-label">
 				<input type="checkbox" id="closed-loop" checked> Closed-loop
 			</label>
@@ -63,6 +69,12 @@
 		</div>
 		<div class="panel">
 			<h3>Stirrer</h3>
+			<div class="device-status-row">
+				<span id="stirrer-status" class="status-disconnected">Disconnected</span>
+				<span id="stirrer-firmware" class="device-firmware"></span>
+				<button id="btn-stirrer-connect">Connect</button>
+				<button id="btn-stirrer-disconnect" disabled>Disconnect</button>
+			</div>
 			<label>Speed [%]
 				<div class="slider-row">
 					<input type="range" id="stirrer-speed" min="0" max="100" step="1">