Add sanity check for server configuration

lib/helpers/sanity_checks.dart

@@ -0,0 +1,66 @@
+/*
+ * This file is part of wger Workout Manager <https://github.com/wger-project>.
+ * Copyright (c) 2025 wger Team
+ *
+ * wger Workout Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ */
+
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+
+/// Checks if the server's reverse proxy is configured correctly
+/// by comparing pagination URLs with the base URL domain.
+Future<SanityCheckResult> checkServerPaginationUrls({
+  required String baseUrl,
+  required String token,
+  http.Client? client,
+}) async {
+  final httpClient = client ?? http.Client();
+
+  try {
+    final baseUri = Uri.parse(baseUrl);
+    final expectedHost = baseUri.host;
+    final expectedScheme = baseUri.scheme;
+
+    final response = await httpClient.get(
+      Uri.parse('$baseUrl/api/v2/exercise/?limit=1'),
+      headers: {
+        'Authorization': 'Token $token',
+        'Accept': 'application/json',
+      },
+    );
+
+    if (response.statusCode != 200) {
+      return const SanityCheckResult(isValid: false);
+    }
+
+    final data = jsonDecode(response.body) as Map<String, dynamic>;
+    final nextUrl = data['next'] as String?;
+
+    if (nextUrl == null) {
+      return const SanityCheckResult(isValid: true);
+    }
+
+    final nextUri = Uri.parse(nextUrl);
+
+    if (nextUri.host != expectedHost || nextUri.scheme != expectedScheme) {
+      return const SanityCheckResult(isValid: false);
+    }
+
+    return const SanityCheckResult(isValid: true);
+  } catch (e) {
+    return const SanityCheckResult(isValid: false);
+  }
+}
+
+/// Result of a server sanity check
+class SanityCheckResult {
+  const SanityCheckResult({
+    required this.isValid,
+  });
+
+  final bool isValid;
+}

lib/l10n/app_en.arb

@@ -1136,5 +1136,21 @@
   "addToOpenFoodFacts": "Add to Open Food Facts",
   "@addToOpenFoodFacts": {
     "description": "Label shown as the clickable link to go on Open Food Facts"
+  },
+  "serverConfigIssueTitle": "Server Configuration Issue",
+  "@serverConfigIssueTitle": {
+   "description": "Title of the dialog shown when server misconfiguration is detected"
+   },
+   "serverConfigIssueMessage": "Server misconfiguration detected, headers are not being passed correctly, please consult the documentation.",
+   "@serverConfigIssueMessage": {
+   "description": "Message explaining server misconfiguration and linking to documentation"
+   },
+   "understand": "I understand",
+   "@understand": {
+   "description": "Button label to acknowledge and dismiss the server configuration warning"
+   },
+   "viewDocumentation": "View documentation",
+   "@viewDocumentation": {
+   "description": "Button label to open the documentation about server configuration"
    }
 }

lib/providers/auth.dart

@@ -29,6 +29,7 @@ import 'package:package_info_plus/package_info_plus.dart';
 import 'package:version/version.dart';
 import 'package:wger/core/exceptions/http_exception.dart';
 import 'package:wger/helpers/consts.dart';
+import 'package:wger/helpers/sanity_checks.dart';
 import 'package:wger/helpers/shared_preferences.dart';
 
 import 'helpers.dart';
@@ -53,6 +54,8 @@ class AuthProvider with ChangeNotifier {
   PackageInfo? applicationVersion;
   Map<String, String> metadata = {};
   AuthState state = AuthState.loggedOut;
+  bool _serverConfigWarning = false;
+  bool get serverConfigWarning => _serverConfigWarning;
 
   static const MIN_APP_VERSION_URL = 'min-app-version';
   static const SERVER_VERSION_URL = 'version';
@@ -66,6 +69,12 @@ class AuthProvider with ChangeNotifier {
     this.client = client ?? http.Client();
   }
 
+  /// Clear the server config warnings
+  void clearServerConfigWarning() {
+    _serverConfigWarning = false;
+    notifyListeners();
+  }
+
   /// flag to indicate that the application has successfully loaded all initial data
   bool dataInit = false;
 
@@ -192,6 +201,21 @@ class AuthProvider with ChangeNotifier {
       return LoginActions.update;
     }
 
+    // Check server configuration sanity
+    try {
+      final sanityCheck = await checkServerPaginationUrls(
+        baseUrl: serverUrl,
+        token: token!,
+      );
+
+      if (!sanityCheck.isValid) {
+        _serverConfigWarning = true;
+        notifyListeners();
+      }
+    } catch (e) {
+      _logger.info('Sanity check error: $e');
+    }
+
     // Log user in
     state = AuthState.loggedIn;
     notifyListeners();

lib/screens/auth_screen.dart

@@ -30,6 +30,7 @@ import 'package:wger/widgets/auth/email_field.dart';
 import 'package:wger/widgets/auth/password_field.dart';
 import 'package:wger/widgets/auth/server_field.dart';
 import 'package:wger/widgets/auth/username_field.dart';
+import 'package:wger/widgets/core/server_config_warning_dialog.dart';
 
 import '../providers/auth.dart';
 
@@ -206,6 +207,17 @@ class _AuthCardState extends State<AuthCard> {
         );
         return;
       }
+
+      if (context.mounted && res == LoginActions.proceed) {
+        final authProvider = context.read<AuthProvider>();
+        if (authProvider.serverConfigWarning) {
+          if (context.mounted) {
+            showServerConfigWarning(context);
+            authProvider.clearServerConfigWarning();
+          }
+        }
+      }
+
       if (context.mounted) {
         setState(() {
           _isLoading = false;

lib/widgets/core/server_config_warning_dialog.dart

@@ -0,0 +1,48 @@
+/*
+ * This file is part of wger Workout Manager <https://github.com/wger-project>.
+ * Copyright (c) 2025 wger Team
+ *
+ * wger Workout Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ */
+
+import 'package:flutter/material.dart';
+import 'package:wger/helpers/misc.dart';
+import 'package:wger/l10n/generated/app_localizations.dart';
+
+/// Shows a warning dialog when server misconfiguration is detected
+void showServerConfigWarning(BuildContext context) {
+  final i18n = AppLocalizations.of(context);
+
+  showDialog(
+    context: context,
+    builder: (context) => AlertDialog(
+      title: Row(
+        children: [
+          const Icon(Icons.warning, color: Colors.orange, size: 28),
+          const SizedBox(width: 12),
+          Expanded(child: Text(i18n.serverConfigIssueTitle)),
+        ],
+      ),
+      content: Text(i18n.serverConfigIssueMessage),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(context),
+          child: Text(i18n.understand),
+        ),
+        TextButton(
+          onPressed: () {
+            launchURL(
+              'https://wger.readthedocs.io/en/latest/administration/errors.html#wrong-pagination-links',
+              context,
+            );
+            Navigator.pop(context);
+          },
+          child: Text(i18n.viewDocumentation),
+        ),
+      ],
+    ),
+  );
+}