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_de.h

@@ -240,6 +240,7 @@ static const char* const I18N_DE[] = {
     "Tippen zum Überspringen",
     "Route: ",
     "Mit dem Handy scannen:",
+    "Ruhebildschirm: ",
     "Nach Update suchen",
     "Suche nach Updates...",
     "Prüfung fehlgeschlagen. WLAN prüfen.",