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_en.h

@@ -232,6 +232,12 @@ static const char* const I18N_EN[] = {
     "e.g. Home, Work, Holiday home",
 
     "Ground vehicle",
+
+    "Backup & Reset",
+    "Reset settings",
+    "Deletes ALL data from the SD card (settings, logbook, location presets, WiFi, calibration) and restarts the device. The full first-time setup will then run again, just like on the very first boot. This CANNOT be undone!",
+    "Deleting all data...",
+    "Reset failed.",
 };
 
 static_assert(sizeof(I18N_EN) / sizeof(I18N_EN[0]) == (size_t)StringId::COUNT,