feat: update backend OCR parser, web app, mobile app camera/preview UI, tests, and documentation with sample images

test/exif_helper_test.dart

@@ -0,0 +1,24 @@
+import 'dart:io';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:app_pfm_ocr_v2/core/utils/exif_helper.dart';
+
+void main() {
+  test('ExifHelper returns null for non-existent file', () async {
+    final result = await ExifHelper.extractGps('non_existent_file.jpg');
+    expect(result, isNull);
+  });
+
+  test('ExifHelper returns null for plain text files / invalid images', () async {
+    final tempDir = Directory.systemTemp.createTempSync();
+    final tempFile = File('${tempDir.path}/plain_text.txt');
+    tempFile.writeAsStringSync('not a jpg image file data');
+
+    try {
+      final result = await ExifHelper.extractGps(tempFile.path);
+      expect(result, isNull);
+    } finally {
+      tempFile.deleteSync();
+      tempDir.deleteSync();
+    }
+  });
+}