V21: keep master FX session warm on user stop - no 4s VST reload on replay

- stopAllPlayback(fade=true) giữ master FX session (không teardown) → play
  kế tiếp FX xử lý ngay, hết cửa sổ dry ~4s khi reload VST
- per-track FX vẫn teardown khi user stop (VSTi nhiều track nặng CPU)
- idle timeout 120s (FX_RT_WARM_IDLE_MS): ngừng chơi lâu → teardown giải
  phóng CPU/VST
- fxRtStart/fxRtStop/startSubTabPlayback hủy idle timer khi play

app/templates/index.html

@@ -51,7 +51,7 @@
     <script src="/static/js/services/promptTemplateManager.js?v=202607281039"></script>
     <script src="/static/js/services/undoRedoEngine.js?v=202607290941"></script>
     <script src="/static/js/services/chordTheory.js?v=202608211300"></script>
-    <script src="/static/js/app.precompiled.js?v=202608311000" defer></script>
+    <script src="/static/js/app.precompiled.js?v=202608311100" defer></script>
     <link rel="stylesheet" href="/static/css/styles.css?v=202607271016">
     <style>
         :root {