feat: Maestro cycling note animation - quarter, eighth, beamed eighths, sixteenth

The note icon now cycles through four musical note variants during
loading: quarter note → eighth note → beamed eighths → sixteenth note.
Each variant bounces on staff lines while cycling every 800ms.
Idle state shows a static eighth note. No more generic spinner.

frontend/src/components/MaestroButton.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useState, useEffect, useRef } from 'react'
 import type { PromptStructure } from './StructuredPromptEditor'
 
 interface Props {
@@ -19,26 +19,39 @@ interface Props {
   onStructuredResult?: (fields: PromptStructure, mode: 'generate' | 'complete') => void
 }
 
-function NoteIcon({ bouncing }: { bouncing: boolean }) {
+// ── Note Variants ─────────────────────────────────────────────────────
+
+const NOTES = [
+  // Quarter note (♩)
+  'M5.5 13 C4 13 3 12 3 11.5 C3 11 4 10.5 5.5 10.5 C7 10.5 8 11 8 11.5 C8 12 7 13 5.5 13 Z M7.8 11 L7.8 4',
+  // Eighth note (♪)
+  'M5.5 13 C4 13 3 12 3 11.5 C3 11 4 10.5 5.5 10.5 C7 10.5 8 11 8 11.5 C8 12 7 13 5.5 13 Z M7.8 11 L7.8 4 C10 5.5 9.5 7.5 7.8 8',
+  // Beamed eighths (♫)
+  'M3 13 C1.5 13 0.5 12 0.5 11.5 C0.5 11 1.5 10.5 3 10.5 C4.5 10.5 5.5 11 5.5 11.5 C5.5 12 4.5 13 3 13 Z M5.3 11 L5.3 4 M10 12.5 C8.5 12.5 7.5 11.5 7.5 11 C7.5 10.5 8.5 10 10 10 C11.5 10 12.5 10.5 12.5 11 C12.5 11.5 11.5 12.5 10 12.5 Z M12.3 11 L12.3 4 M5.3 4 L12.3 4',
+  // Sixteenth note (♬)
+  'M5.5 13 C4 13 3 12 3 11.5 C3 11 4 10.5 5.5 10.5 C7 10.5 8 11 8 11.5 C8 12 7 13 5.5 13 Z M7.8 11 L7.8 4 M7.8 7 C10 6 9.5 5.5 7.8 5 M7.8 9 C10 8 9.5 7.5 7.8 7',
+]
+
+const STAFF_LINES = (
+  <>
+    <line x1="2" y1="5" x2="16" y2="5" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
+    <line x1="2" y1="8" x2="16" y2="8" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
+    <line x1="2" y1="11" x2="16" y2="11" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
+    <line x1="2" y1="14" x2="16" y2="14" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
+  </>
+)
+
+function NoteIcon({ noteIndex, bouncing }: { noteIndex: number; bouncing: boolean }) {
   return (
     <svg
       width="14"
       height="14"
-      viewBox="0 0 18 18"
+      viewBox="0 0 16 16"
       className={bouncing ? 'animate-note-bounce' : ''}
       aria-hidden="true"
     >
-      {/* Staff lines */}
-      <line x1="2" y1="5" x2="16" y2="5" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
-      <line x1="2" y1="8" x2="16" y2="8" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
-      <line x1="2" y1="11" x2="16" y2="11" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
-      <line x1="2" y1="14" x2="16" y2="14" stroke="currentColor" strokeWidth="0.7" opacity="0.4" />
-      {/* Note head */}
-      <ellipse cx="5.5" cy="13" rx="2.5" ry="2" fill="currentColor" transform="rotate(-15 5.5 13)" />
-      {/* Stem */}
-      <line x1="7.8" y1="12.5" x2="7.8" y2="4" stroke="currentColor" strokeWidth="1.2" />
-      {/* Flag */}
-      <path d="M7.8 4 C10 5.5, 9.5 7.5, 7.8 8" fill="none" stroke="currentColor" strokeWidth="1" />
+      {STAFF_LINES}
+      <path d={NOTES[noteIndex]} fill="none" stroke="currentColor" strokeWidth="0.8" />
     </svg>
   )
 }
@@ -52,6 +65,26 @@ export default function MaestroButton({
   onStructuredResult,
 }: Props) {
   const [isMaestroing, setIsMaestroing] = useState(false)
+  const [noteIndex, setNoteIndex] = useState(1) // Start on eighth note (♪)
+  const intervalRef = useRef<ReturnType<typeof setInterval> | null>(null)
+
+  // Cycle through note variants while loading
+  useEffect(() => {
+    if (isMaestroing) {
+      intervalRef.current = setInterval(() => {
+        setNoteIndex(prev => (prev + 1) % NOTES.length)
+      }, 800) // Change note every 800ms (slightly slower than bounce)
+    } else {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current)
+        intervalRef.current = null
+      }
+      setNoteIndex(1) // Reset to eighth note when idle
+    }
+    return () => {
+      if (intervalRef.current) clearInterval(intervalRef.current)
+    }
+  }, [isMaestroing])
 
   const handleMaestro = async () => {
     setIsMaestroing(true)
@@ -109,7 +142,7 @@ export default function MaestroButton({
         : "Let Maestro complete your prompt with musical details"
       }
     >
-      <NoteIcon bouncing={isMaestroing} />
+      <NoteIcon noteIndex={noteIndex} bouncing={isMaestroing} />
       <span className="truncate">
         {isMaestroing
           ? (mode === 'generate' ? 'Dreaming...' : 'Composing...')