Add Closed-loop/Open-loop heater control mode

Closed-loop (default, checkbox checked): TC has exclusive control over
the heater; temp-soll and heatrate-soll sliders are active; direct
heater power entry is disabled. TC now stays enabled at server startup
and whenever closed-loop mode is active — SudTask no longer disables it
on IDLE/DONE, so adjusting temp-soll manually with no sud running
immediately drives the heater via the TC.

Open-loop (checkbox unchecked, only selectable when paused/stopped):
TC is disconnected from the heater; only the heater-power slider is
active. Switching back to closed-loop resets power_soll to 0 and
re-enables the TC.

On play, the mode is automatically reset to closed-loop.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SH2D4LRuCnhLSzoYerAfJX

web/index.html

@@ -43,6 +43,9 @@
 	</div>
 	<div class="panel">
 		<h3>Heater</h3>
+		<label class="checkbox-label">
+			<input type="checkbox" id="closed-loop" checked> Closed-loop
+		</label>
 		<label>Power [W]
 			<div class="slider-row">
 				<input type="range" id="heater-power" min="0" max="100" step="1">