feat: SOS (experimental), issue #56: a countdown, then the position to chosen channels and contacts, repeated until someone answers

L1: Tools > SOS, Back held 3 s. L2: the SOS app, the top button held 5 s
opens a slider over everything, the lock screen and a PIN too. Off by default.
The ACK word stops the repeats; who answered, what they wrote and when show on
Answered, and whoever answered needs no word after that. Beep plays
... --- ... at full volume until it's ended. While one runs the screen goes off
by 30 s, the GPS stays awake past its duty cycle until nothing more will be
sent, the repeater and the L2's WiFi rest. A received SOS gets a banner and a
place on the map, past Track loc, for 24 h.

The lock screen's slider restyled with it, shared as slideBar() / slidePoll();
the L2's Favourites and SOS pickers share pickContacts(). The engine is
ui-core/SosEngine.h, tested on the host. The message's format isn't final.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/HomeScreen.h

@@ -35,6 +35,7 @@ static const App APP_LIST[] = {
   { 'D', UI_SYMBOL_CHART,    "Diagnostics", onOpenDiag,      false },
   { 'L', UI_SYMBOL_BULB,     "Flashlight",  onOpenFlashlight, false },
   { 'P', LV_SYMBOL_IMAGE,    "Gallery",     onOpenGallery,   false },
+  { 'O', LV_SYMBOL_WARNING,  "SOS",         onOpenSos,       false },
 };
 static const int APP_COUNT = sizeof(APP_LIST) / sizeof(APP_LIST[0]);
 static const int PER_PAGE = 6;