Fix copy button symbol to use more standard Unicode character

- Replaced ⎘ (U+2398) with 📋 (clipboard emoji) for better cross-platform compatibility
- More recognizable and widely supported symbol

frontend/app/admin/availability/page.tsx

@@ -292,7 +292,7 @@ export default function AdminAvailabilityPage() {
                         style={styles.copyFromButton}
                         title="Copy to other days"
                       >
-                        ⎘
+                        📋
                       </button>
                     )}
                   </div>