feat: wire split panel + fix TS ref callback void return

ImportPanel.tsx

@@ -92,7 +92,7 @@ export default function ImportPanel({ onGlbReady }: ImportPanelProps) {
               </>
             )}
             <input
-              ref={el => inputRefs.current[i] = el}
+              ref={el => { inputRefs.current[i] = el; }}
               type="file" accept="image/png,image/jpeg,image/webp"
               style={{ display: 'none' }}
               onChange={e => handleFile(i, e.target.files?.[0] ?? null)}