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

web/style.css

@@ -350,7 +350,6 @@ header#connection-bar {
     font-size: 0.9em;
 }
 .device-status-row button { padding: 0.15em 0.6em; font-size: 0.9em; }
-.device-firmware { color: var(--text-muted); }
 
 /* --- Hidden --- */
 .hidden { display: none !important; }