fix(midi): route MIDI live via masterBus, fix media explorer drag, add action logging

- TrackInstrument: gate native live path (SF/VSTi WASAPI bridge bypassed
  masterBus -> mastering FX + main out VU dead). Fallback to SonicSF WASM
  through masterBus.input; native kept for offline render only.
- MEDIA_LIBRARY_SAMPLES: rename MIDI_Loop_02_Bass/03_Lead/05_Bass to match
  actual /static/midi files (drag was 404 -> parse error).
- Add SonicAppLogger (console + POST /api/v1/logs) wired to: instrument load
  error, VSTi autosample fail, MIDI item -> mastering fx chain, play -> main
  out, drag MIDI errors (media explorer + window explorer).
- Add backend /api/v1/logs endpoint (server-side log, no DB).

app/static/js/services/appLogger.js

@@ -0,0 +1,40 @@
+// SonicForge App Logger — ghi hành động quan trọng của client (console + POST
+// /api/v1/logs để server log). Fire-and-forget: không throw, không block UI.
+// Các category dùng chung:
+//   INSTRUMENT  — lỗi load instrument (soundfont select/load fail)
+//   VSTI        — VSTi load/autosample/preview fail
+//   MIDI_ITEM   — MIDI item qua mastering fx chain / main out (playback)
+//   DRAG_MIDI   — lỗi drag MIDI từ MEDIA EXPLORER hoặc window explorer
+window.SonicAppLogger = window.SonicAppLogger || (function () {
+  function base() {
+    return window.API_BASE_URL || window.location.origin || '';
+  }
+  function emit(category, level, message, data) {
+    try {
+      var line = '[AppLog][' + category + '][' + level + '] ' + message;
+      if (data !== undefined && data !== null) {
+        try { line += ' ' + JSON.stringify(data); } catch (e) {}
+      }
+      if (level === 'error') console.error(line); else if (level === 'warn') console.warn(line); else console.log(line);
+      try {
+        fetch(base() + '/api/v1/logs', {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            category: category,
+            level: level,
+            message: message,
+            data: data !== undefined ? data : null,
+            url: window.location ? window.location.href : '',
+            ts: new Date().toISOString()
+          })
+        }).catch(function () {});
+      } catch (e) {}
+    } catch (e) {}
+  }
+  return {
+    info: function (category, message, data) { emit(category, 'info', message, data); },
+    warn: function (category, message, data) { emit(category, 'warn', message, data); },
+    error: function (category, message, data) { emit(category, 'error', message, data); }
+  };
+})();

app/static/js/services/trackInstrument.js

@@ -22,7 +22,15 @@ window.TrackInstrument = window.TrackInstrument || {};
 
     // Loại engine native cho track: 'sf' (soundfont), 'vst3'/'vst2' (VSTi có
     // plugin_id) hay null (không native được → fallback WASM/autosample).
+    // T15-regression fix: native live (SF/VSTi qua WASAPI bridge) phát thẳng ra
+    // OS device — BỎ QUA WebAudio masterBus → mastering FX + Main out VU không
+    // nhận tín hiệu. Gate native live path: luôn fallback WASM (SonicSF/
+    // autosample) qua masterBus.input. Native vẫn dùng cho OFFLINE render
+    // (scheduleNativeSfItem / /api/v1/native/render) — không đi qua hàm này.
+    // ponytail: re-enable live native khi C++ bridge có capture/readback →
+    // set window.__SONICFORGE_NATIVE_LIVE = true.
     TrackInstrument.prototype._nativeKind = function () {
+        if (!window.__SONICFORGE_NATIVE_LIVE) return null;
         try {
             if (this.sfId) return 'sf';
             if (window.SonicNativeAudio && this.synthEngine && this.synthEngine.plugin_id) {