feat(editor): surface quest-chain validation issues to user via toast

validateChains warnings were only going to the log file. Most users
don't tail the log while editing, so a broken chain (quest pointing at
a deleted nextQuestId, circular dependency) would only be discovered
when testing in-game. Now also shows a 5s toast with the issue count.

tools/editor/editor_app.cpp

@@ -1105,6 +1105,10 @@ void EditorApp::exportZone(const std::string& outputDir) {
         if (!questEditor_.validateChains(chainErrors)) {
             for (const auto& err : chainErrors)
                 LOG_WARNING("Quest chain issue: ", err);
+            // Surface chain issues to the user — silently logging means most
+            // users won't notice a broken chain until they test in-game.
+            showToast("Quest chains have " + std::to_string(chainErrors.size()) +
+                      " issue(s) — see log", 5.0f);
         }
     }