feat: Maestro loading animation - bouncing music note on staff

Replaced generic spinning character with a custom SVG eighth note
bouncing against four staff lines. Added note-bounce keyframe to
Tailwind config (0.6s ease-in-out infinite). More on-brand for a
music tool than a generic spinner.

frontend/tailwind.config.js

@@ -19,9 +19,14 @@ export default {
           '0%': { transform: 'translateX(0)' },
           '100%': { transform: 'translateX(-50%)' },
         },
+        'note-bounce': {
+          '0%, 100%': { transform: 'translateY(0)' },
+          '50%': { transform: 'translateY(-3px)' },
+        },
       },
       animation: {
         'marquee': 'marquee-scroll linear infinite',
+        'note-bounce': 'note-bounce 0.6s ease-in-out infinite',
       },
     },
   },