Refactor code structure for improved readability and maintainability

Co-authored-by: Copilot <copilot@github.com>

flutter/lib/core/auth/jwt_decoder.dart

@@ -0,0 +1,26 @@
+import 'dart:convert';
+
+/// Decodes a JWT token payload without verifying signature.
+/// Returns the decoded claims or an empty map on failure.
+Map<String, dynamic> decodeJwtPayload(String token) {
+  try {
+    final parts = token.split('.');
+    if (parts.length != 3) return {};
+    // Normalize base64url to standard base64.
+    final payload = base64Url.normalize(parts[1]);
+    final decoded = utf8.decode(base64Url.decode(payload));
+    return json.decode(decoded) as Map<String, dynamic>;
+  } catch (_) {
+    return {};
+  }
+}
+
+/// Returns the role claim from a JWT token. Defaults to 'user'.
+String jwtRole(String? token) {
+  if (token == null || token.isEmpty) return 'user';
+  final claims = decodeJwtPayload(token);
+  return claims['role'] as String? ?? 'user';
+}
+
+/// Returns true if the JWT token contains role == 'admin'.
+bool jwtIsAdmin(String? token) => jwtRole(token) == 'admin';