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/sd_storage.cpp

@@ -143,4 +143,35 @@ void logEvent(const char* csvLine) {
     f.close();
 }
 
+bool deleteDirectoryRecursive(const char* path) {
+    File dir = SD.open(path);
+    if (!dir || !dir.isDirectory()) {
+        if (dir) dir.close();
+        return false;
+    }
+
+    // Gleiches vorsichtiges "entry.name() koennte relativ ODER absolut
+    // sein"-Muster wie in flight_logbook.cpp::resetAllData() - anders als
+    // dort aber mit echter Rekursion in Unterordner (statt sie zu
+    // ueberspringen), da der Flightradar-Ordner welche enthaelt (logs/,
+    // screenshots/).
+    File entry = dir.openNextFile();
+    while (entry) {
+        bool isDir = entry.isDirectory();
+        String name = String(entry.name());
+        entry.close();
+
+        String fullPath = name.startsWith("/") ? name : String(path) + "/" + name;
+        if (isDir) {
+            deleteDirectoryRecursive(fullPath.c_str());
+        } else {
+            SD.remove(fullPath);
+        }
+        entry = dir.openNextFile();
+    }
+    dir.close();
+
+    return SD.rmdir(path);
+}
+
 }