fix(auth): remove auto-signout on 401 in AuthInterceptor

The AuthInterceptor was calling signOut() on any 401 error, which caused
the theme toggle to trigger logout when the preferences API returned 401.
Now 401 errors propagate to calling code for graceful handling.

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

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

lib/core/api/api_interceptors.dart

@@ -38,16 +38,9 @@ class AuthInterceptor extends Interceptor {
 
   @override
   void onError(DioException err, ErrorInterceptorHandler handler) {
-    // Skip auth error handling for LAN development
-    if (!AppConfig.requiresAuth) {
-      handler.next(err);
-      return;
-    }
-
-    if (err.response?.statusCode == 401) {
-      // Token expired - trigger re-authentication
-      _ref.read(authProvider.notifier).signOut();
-    }
+    // Don't auto-signout on 401 - let calling code handle auth errors gracefully.
+    // Auto-signout was causing issues (e.g., theme toggle triggering logout when
+    // preferences API returned 401).
     handler.next(err);
   }
 }