Add idle screensaver feature; fix OTA release asset filename

New "Screensaver" toggle in Menu > System (off by default). When the
inactivity timeout fires with it enabled, the display shows a dimmed
starfield with a large centered clock instead of turning the
backlight fully off; a tap wakes it and redraws normally. Off keeps
the existing behavior unchanged.

Also fixes OtaUpdate::checkForUpdate() to look for the release asset
"CYD-flightradar.bin" (the actual filename used by the release
workflow) instead of the placeholder "firmware.bin" it was checking
for before.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n_es.h

@@ -240,6 +240,7 @@ static const char* const I18N_ES[] = {
     "Toca para saltar",
     "Ruta: ",
     "Escanea con el móvil:",
+    "Salvapantallas: ",
     "Buscar actualización",
     "Buscando actualizaciones...",
     "Error al comprobar. Revisa tu conexión WiFi.",