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.h

@@ -282,6 +282,21 @@ enum class StringId : uint8_t {
     // Hoehen-Farbstufen erscheinen.
     LEGEND_GROUND_VEHICLE,
 
+    // Neue "Sicherung & Reset"-Unterseite im System-Menue (fasst die
+    // bisher einzeln im System-Menue stehenden Backup/Restore-Buttons
+    // zusammen und ergaenzt einen dritten Punkt "Einstellungen
+    // zuruecksetzen" - ein kompletter Werksreset fuer Entwickler/Tester,
+    // der den gesamten Flightradar-Ordner von der SD-Karte loescht und
+    // neu startet, siehe settings_backup.cpp::factoryReset()). Titel und
+    // Button-Label teilen sich MENU_BACKUP_RESET, genau wie
+    // MENU_CATEGORY_SYSTEM sowohl fuer den Menue-Button als auch den
+    // Bildschirmtitel der System-Seite verwendet wird.
+    MENU_BACKUP_RESET,
+    MENU_FACTORY_RESET,
+    MENU_FACTORY_RESET_WARNING_BODY,
+    MENU_FACTORY_RESET_DELETING,
+    MENU_FACTORY_RESET_FAILED,
+
     COUNT
 };