fix: MIDI Rearrange — selected item triggers dedicated tool flow

- midiExtractor.js: extractSelectedMIDIContext with note name conversion
- aiGateway.js: REARRANGE_TOOL_SPEC + buildRearrangeMessage
- dawCommandDispatcher.js: register REARRANGE_MIDI_MELODY command
- app.jsx: handleAISend detects selected MIDI item → rearrange flow
- rearrangeMidiMelody handler places A/B track below source track

app/templates/index.html

@@ -20,6 +20,7 @@
     <script src="/static/js/services/dawCommandDispatcher.js?v=202607271016"></script>
     <script src="/static/js/services/pianoRollTabService.js?v=202607272044"></script>
     <script src="/static/js/services/ghostNoteExtractor.js?v=202607271727"></script>
+    <script src="/static/js/services/midiExtractor.js?v=202607281052"></script>
     <script src="/static/js/services/promptTemplateManager.js?v=202607281039"></script>
     <script src="/static/js/app.precompiled.js?v=202607271245" defer></script>
     <link rel="stylesheet" href="/static/css/styles.css?v=202607271016">