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/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');
+}