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/static/js/services/aiGateway.js

@@ -88,6 +88,64 @@ const AIGateway = (function() {
     }
   }];
 
+  const REARRANGE_TOOL_SPEC = {
+    type: 'function',
+    function: {
+      name: 'rearrange_midi_melody',
+      description: 'Accepts source MIDI notes and rearranges/re-harmonizes them into a new musical variation while preserving the core melody. Use this when the user asks to rearrange, remix, or create variations of an existing MIDI item.',
+      parameters: {
+        type: 'object',
+        properties: {
+          rearrange_title: { type: 'string', description: 'Title/description of the rearranged version (e.g. "Jazz Variation", "Dark Orchestral Remix")' },
+          soundfont_id: { type: 'string', default: 'generaluser_gs' },
+          soundfont_bank: { type: 'integer', default: 0 },
+          soundfont_program: { type: 'integer', default: 0 },
+          rearranged_notes: {
+            type: 'array',
+            description: 'Array of rearranged MIDI notes. Keep the same total_duration_beats as the original unless user explicitly requests length change.',
+            items: {
+              type: 'object',
+              properties: {
+                pitch: { type: 'integer', description: 'MIDI note pitch 0-127 (C4=60)' },
+                start_beat: { type: 'number', description: 'Note start position in beats from 0.0' },
+                duration_beats: { type: 'number', description: 'Note length in beats (quarter=1.0)' },
+                velocity: { type: 'number', description: 'Velocity 0.0-1.0' }
+              },
+              required: ['pitch', 'start_beat', 'duration_beats', 'velocity']
+            }
+          }
+        },
+        required: ['rearrange_title', 'rearranged_notes']
+      }
+    }
+  };
+
+  function buildRearrangeMessage(prompt, sourceContext) {
+    const notesJson = JSON.stringify(sourceContext.notes, null, 2);
+    return [
+      { role: 'system', content: `You are a professional music arranger. Your task is to rearrange/re-harmonize the provided MIDI notes based on the user's request.
+
+SOURCE MIDI CONTEXT:
+- Track: ${sourceContext.track_name}
+- Item: ${sourceContext.item_name}
+- Duration: ${sourceContext.duration_bars} bars (${sourceContext.total_beats} beats)
+- BPM: ${sourceContext.bpm}
+- Total notes: ${sourceContext.total_notes}
+
+SOURCE NOTES (pitch, note_name, start_beat, duration_beats, velocity):
+${notesJson}
+
+RULES:
+1. Use the rearrange_midi_melody tool to return the rearranged notes.
+2. PRESERVE the core melodic outline and overall structure unless the user explicitly asks for a complete transformation.
+3. Keep the total duration (${sourceContext.total_beats} beats) the same unless user requests a different length.
+4. The rearranged_notes array MUST contain notes with pitch (0-127), start_beat (0.0 to ${sourceContext.total_beats}.0), duration_beats, and velocity (0.0-1.0).
+5. Start beats should remain within 0-${sourceContext.total_beats} range.
+6. You may add, remove, or modify notes to achieve the requested style.` },
+      { role: 'user', content: `User request: ${prompt}\n\nRearrange the source MIDI notes above according to this request. Return the result via the rearrange_midi_melody tool.` }
+    ];
+  }
+
   function parseOrigin(urlStr) {
     try { const u = new URL(urlStr); return `${u.protocol}//${u.hostname}${u.port ? ':'+u.port : ''}`; } catch (_) { return null; }
   }
@@ -303,6 +361,8 @@ Ví dụ: "Hãy chọn và copy từ bar 4 đến bar 12 của track 1 sau đó
 
   return {
     DEFAULT_TOOLS,
+    REARRANGE_TOOL_SPEC,
+    buildRearrangeMessage,
     callLLM,
     extractFunctionCalls,
     buildUserMessage,