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.h

@@ -316,6 +316,11 @@ enum class StringId : uint8_t {
     // noetig.
     WEBUI_QR_HINT,
 
+    // Ruhebildschirm bei Inaktivitaets-Timeout (Menue > System >
+    // "Ruhebildschirm", siehe SettingsStore::screensaverEnabled() und
+    // main.cpp).
+    MENU_SCREENSAVER,
+
     // OTA-Firmware-Update ueber WLAN (Menue > System > "Nach Update
     // suchen", siehe menu_screen.cpp::runOtaUpdateScreen() und
     // ota_update.h/.cpp).