feat(app): add manual entry and sharing flows

doc/03_architecture/naver_url_processing_architecture.md

@@ -278,12 +278,26 @@ class MockHttpClient extends Mock implements Client {}
 ```dart
 // lib/core/constants/api_keys.dart
 class ApiKeys {
-  static const String naverClientId = String.fromEnvironment('NAVER_CLIENT_ID');
-  static const String naverClientSecret = String.fromEnvironment('NAVER_CLIENT_SECRET');
-  
+  static const String _encodedClientId =
+      String.fromEnvironment('NAVER_CLIENT_ID', defaultValue: '');
+  static const String _encodedClientSecret =
+      String.fromEnvironment('NAVER_CLIENT_SECRET', defaultValue: '');
+
+  static String get naverClientId => _decode(_encodedClientId);
+  static String get naverClientSecret => _decode(_encodedClientSecret);
+
   static bool areKeysConfigured() {
     return naverClientId.isNotEmpty && naverClientSecret.isNotEmpty;
   }
+
+  static String _decode(String value) {
+    if (value.isEmpty) return '';
+    try {
+      return utf8.decode(base64.decode(value));
+    } on FormatException {
+      return value;
+    }
+  }
 }
 ```
 
@@ -397,4 +411,4 @@ class ErrorReporter {
 ### 10.2 하위 호환성
 - 기존 NaverMapParser는 그대로 유지
 - 새로운 기능은 옵트인 방식으로 제공
-- 점진적 마이그레이션 지원
+- 점진적 마이그레이션 지원