fix(media-explorer): bridge preview channel 15 + NOTE_ON timing; synth dropdown portal

app/static/js/services/unifiedMidiRouter.js

@@ -3,7 +3,7 @@
 // bridge connected -> NativeBridgeService.dispatchMidiEvent (Rust SHM -> C++ bridge)
 // bridge down      -> onFallback callback (app.jsx wires SonicSF/Carla path).
 (function () {
-  var MELODIC_CHANNELS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15]; // skip 9 (percussion)
+  var MELODIC_CHANNELS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14]; // skip 9 (percussion); 15 reserved for Media Explorer preview
 
   var router = {
     bridgeConnected: false,
@@ -24,6 +24,9 @@
 
     /** trackId -> MIDI channel; percussion (bank 128) -> ch 9. */
     allocateChannel: function (trackId, isPercussion) {
+      // Media Explorer preview dùng channel 15 cố định — tránh đè channel
+      // melodic track (0-8,10-14) / percussion (9) đang giữ instrument.
+      if (trackId === 'media_explorer') return 15;
       if (this._channels[trackId]) return this._channels[trackId].ch;
       var ch;
       if (isPercussion) {