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_de.h

@@ -231,6 +231,12 @@ static const char* const I18N_DE[] = {
     "z.B. Zuhause, Arbeit, Ferienhaus",
 
     "Bodenfahrzeug",
+
+    "Sicherung & Reset",
+    "Einstellungen zurücksetzen",
+    "Löscht ALLE Daten von der SD-Karte (Einstellungen, Flugbuch, Standort-Presets, WLAN, Kalibrierung) und startet das Gerät neu. Danach läuft die komplette Ersteinrichtung erneut - wie beim allerersten Start. Das kann NICHT rückgängig gemacht werden!",
+    "Lösche alle Daten...",
+    "Zurücksetzen fehlgeschlagen.",
 };
 
 static_assert(sizeof(I18N_DE) / sizeof(I18N_DE[0]) == (size_t)StringId::COUNT,