Add Backup & Reset sub-page with factory reset (developer feature)

Moves the previously standalone "Backup settings"/"Restore settings"
buttons out of the System menu into a new "Backup & Reset" sub-page
(replaced in the System menu by a single folder button, placed
directly above "Info"), and adds a third entry "Reset settings" - a
factory reset for developers/testers that deletes the entire
Flightradar folder from the SD card (settings, logbook, location
presets, WiFi, calibration - everything) and restarts the device. The
next boot picks up isFirstRun automatically (via the missing
SD_SETTINGS_FILE) and runs the full first-time setup again, exactly
like on the very first boot - no need to pull the SD card.

Since this is irreversible, it requires a full-screen red warning
confirmation first, reusing the same overlay previously built only for
the flight-logbook enable warning (confirmLogbookEnable() generalized
to confirmWarningScreen(), taking title/body StringIds as parameters).

New SdStorage::deleteDirectoryRecursive() and
SettingsBackup::factoryReset() implement the actual deletion + restart.

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

src/i18n_es.h

@@ -231,6 +231,12 @@ static const char* const I18N_ES[] = {
     "ej. Casa, Trabajo, Casa de vacaciones",
 
     "Vehículo terrestre",
+
+    "Copia de seguridad y reinicio",
+    "Reiniciar ajustes",
+    "Elimina TODOS los datos de la tarjeta SD (ajustes, registro de vuelos, ubicaciones guardadas, WiFi, calibración) y reinicia el dispositivo. Después se ejecutará de nuevo la configuración inicial completa, como en el primer arranque. ¡Esto NO se puede deshacer!",
+    "Eliminando todos los datos...",
+    "Error al reiniciar.",
 };
 
 static_assert(sizeof(I18N_ES) / sizeof(I18N_ES[0]) == (size_t)StringId::COUNT,