fix(auth): persist PKCE state in sessionStorage

Store OIDC code_verifier and state in sessionStorage instead of
static memory variables. This fixes the "No code verifier" error
that occurred after Authentik redirect because the Flutter app
restarts and loses in-memory state.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/core/auth/oidc_service_web.dart

@@ -8,6 +8,7 @@ import 'package:dio/dio.dart';
 
 import '../config/app_config.dart';
 import 'oidc_service.dart';
+import 'web_utils.dart' as web_utils;
 
 /// Web implementation of OIDC service using browser redirect flow.
 ///
@@ -25,9 +26,9 @@ class OidcServiceWeb implements OidcService {
   /// Redirect URI for web.
   static String get _redirectUri => '${AppConfig.webBaseUrl}/callback';
 
-  // PKCE state stored during auth flow (in-memory for single-page app)
-  static String? _codeVerifier;
-  static String? _state;
+  // SessionStorage keys for PKCE state (persists across redirect)
+  static const _codeVerifierKey = 'oidc_code_verifier';
+  static const _stateKey = 'oidc_state';
 
   /// Get the authorization URL to redirect the browser to.
   ///
@@ -39,11 +40,15 @@ class OidcServiceWeb implements OidcService {
     final authEndpoint = discovery['authorization_endpoint'] as String;
 
     // Generate PKCE code verifier and challenge
-    _codeVerifier = _generateCodeVerifier();
-    final codeChallenge = _generateCodeChallenge(_codeVerifier!);
+    final codeVerifier = _generateCodeVerifier();
+    final codeChallenge = _generateCodeChallenge(codeVerifier);
 
     // Generate state for CSRF protection
-    _state = _generateRandomString(32);
+    final state = _generateRandomString(32);
+
+    // Store PKCE state in sessionStorage (persists across redirect)
+    web_utils.setSessionStorage(_codeVerifierKey, codeVerifier);
+    web_utils.setSessionStorage(_stateKey, state);
 
     // Build authorization URL
     final params = {
@@ -53,7 +58,7 @@ class OidcServiceWeb implements OidcService {
       'scope': _scopes.join(' '),
       'code_challenge': codeChallenge,
       'code_challenge_method': 'S256',
-      'state': _state,
+      'state': state,
     };
 
     final uri = Uri.parse(authEndpoint).replace(queryParameters: params);
@@ -67,12 +72,21 @@ class OidcServiceWeb implements OidcService {
   /// [code] is the authorization code from the callback URL.
   /// [state] is the state parameter from the callback URL (verified for CSRF).
   Future<OidcTokens> exchangeCode(String code, String state) async {
+    // Retrieve PKCE state from sessionStorage
+    final storedState = web_utils.getSessionStorage(_stateKey);
+    final codeVerifier = web_utils.getSessionStorage(_codeVerifierKey);
+
+    developer.log('Stored state: $storedState, received state: $state', name: 'oidc_web');
+    developer.log('Code verifier present: ${codeVerifier != null}', name: 'oidc_web');
+
     // Verify state matches
-    if (_state == null || state != _state) {
+    if (storedState == null || state != storedState) {
+      _clearPkceState();
       throw OidcException('State mismatch - possible CSRF attack');
     }
 
-    if (_codeVerifier == null) {
+    if (codeVerifier == null) {
+      _clearPkceState();
       throw OidcException('No code verifier - flow not started properly');
     }
 
@@ -91,7 +105,7 @@ class OidcServiceWeb implements OidcService {
           'client_id': AppConfig.authClientId,
           'redirect_uri': _redirectUri,
           'code': code,
-          'code_verifier': _codeVerifier,
+          'code_verifier': codeVerifier,
         },
         options: Options(
           contentType: Headers.formUrlEncodedContentType,
@@ -102,8 +116,7 @@ class OidcServiceWeb implements OidcService {
       developer.log('Token exchange successful', name: 'oidc_web');
 
       // Clear stored PKCE state
-      _codeVerifier = null;
-      _state = null;
+      _clearPkceState();
 
       return OidcTokens(
         accessToken: data['access_token'] as String,
@@ -115,14 +128,17 @@ class OidcServiceWeb implements OidcService {
       );
     } on DioException catch (e) {
       developer.log('Token exchange failed: $e', name: 'oidc_web');
+      _clearPkceState();
       throw OidcException('Token exchange failed: ${e.message}');
-    } finally {
-      // Clear PKCE state on error too
-      _codeVerifier = null;
-      _state = null;
     }
   }
 
+  /// Clear PKCE state from sessionStorage.
+  void _clearPkceState() {
+    web_utils.removeSessionStorage(_codeVerifierKey);
+    web_utils.removeSessionStorage(_stateKey);
+  }
+
   /// Not used on web - use [getAuthorizationUrl] and [exchangeCode] instead.
   @override
   Future<OidcTokens> signIn() async {

lib/core/auth/web_utils_stub.dart

@@ -15,3 +15,18 @@ String getCurrentUrl() {
 void replaceUrl(String url) {
   throw UnsupportedError('replaceUrl is only supported on web');
 }
+
+/// Store a value in sessionStorage (no-op on non-web).
+void setSessionStorage(String key, String value) {
+  throw UnsupportedError('setSessionStorage is only supported on web');
+}
+
+/// Get a value from sessionStorage (no-op on non-web).
+String? getSessionStorage(String key) {
+  throw UnsupportedError('getSessionStorage is only supported on web');
+}
+
+/// Remove a value from sessionStorage (no-op on non-web).
+void removeSessionStorage(String key) {
+  throw UnsupportedError('removeSessionStorage is only supported on web');
+}

lib/core/auth/web_utils_web.dart

@@ -17,3 +17,18 @@ String getCurrentUrl() {
 void replaceUrl(String url) {
   web.window.history.replaceState(null, '', url);
 }
+
+/// Store a value in sessionStorage.
+void setSessionStorage(String key, String value) {
+  web.window.sessionStorage.setItem(key, value);
+}
+
+/// Get a value from sessionStorage.
+String? getSessionStorage(String key) {
+  return web.window.sessionStorage.getItem(key);
+}
+
+/// Remove a value from sessionStorage.
+void removeSessionStorage(String key) {
+  web.window.sessionStorage.removeItem(key);
+}