Use platform windows for windowing instead of custom draggable window system

src/ui/ui_piano.h

@@ -0,0 +1,20 @@
+#pragma once
+#include "ui/ui_core.h"
+#include "midi/midi.h"
+
+#define PIANO_FIRST_NOTE  21    // A0
+#define PIANO_LAST_NOTE   108   // C8
+
+struct UI_PianoState {
+    S32 mouse_note;  // MIDI note held by mouse click (-1 = none)
+};
+
+B32        piano_is_black_key(S32 note);
+Clay_Color piano_velocity_color(S32 velocity);
+
+// Render the 88-key piano widget.
+// avail_w: total width for key layout, avail_h: total height for keys
+void ui_piano(UI_PianoState *state, MidiEngine *midi, F32 avail_w, F32 avail_h);
+
+// Update piano mouse input (call after Clay layout is computed)
+void ui_piano_update_input(UI_PianoState *state);