fix(ui): HomeScreen::poll() re-fired shutdown() every tick after the first

_shutdown_init (set true once KEY_ENTER is pressed on the Shutdown home
page) was never cleared, so this branch kept calling _task->shutdown()
on every single poll() tick indefinitely once triggered.

Invisible on real hardware: _board->powerOff() halts the MCU in the
non-restart path, so there's no next tick to matter. But
SimMainBoard::powerOff() is a deliberate no-op (no real hardware to
power off), so a sim instance keeps running after "shutdown" -- and each
repeated shutdown() call re-fires _display->turnOff(), which blacks out
its <canvas> (keyed by simInstanceTag) again on every frame.

This is what made meshcore-solo-site's RESET button unusable after a
device had been shut down: the still-running old instance kept
re-blacking the very canvas a freshly reset instance (same tag, same
canvas element) was trying to render its own boot splash onto -- visible
as a black screen, with the new instance's splash winning a single frame
every so often before being painted over again.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018iubftDmKNWmkNnhJRz8UH

examples/companion_radio/ui-new/UITask.cpp

@@ -534,6 +534,16 @@ public:
 
   void poll() override {
     if (_shutdown_init && !_task->isButtonPressed()) {  // must wait for USR button to be released
+      // _shutdown_init is never cleared elsewhere -- on real hardware
+      // that's harmless because _board->powerOff() halts the MCU, so
+      // there is no next poll() tick to matter. In the sim, powerOff()
+      // is a deliberate no-op (no real hardware to power off, see
+      // SimMainBoard.h), so without this the instance keeps running and
+      // this branch re-fires shutdown() -> _display->turnOff() on every
+      // single tick forever, repeatedly blacking out its canvas -- which
+      // fights with a freshly reset instance's own boot splash trying to
+      // render onto that same (simInstanceTag-keyed) canvas element.
+      _shutdown_init = false;
       _task->shutdown();
     }
   }