feat: toolbar sections as dropdown menus

Replace horizontal toolbar sections with dropdown buttons (I/O, Gates,
Components). Each opens a dropdown menu on click, keeping the toolbar
clean and compact. Dropdowns close on outside click or after selecting
a gate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/components.js

@@ -286,6 +286,8 @@ export function updateComponentButtons() {
         btn.addEventListener('click', (e) => {
             e.stopPropagation();
             state.placingGate = `COMPONENT:${comp.id}`;
+            // Close dropdown
+            document.querySelectorAll('.toolbar-dropdown.open').forEach(d => d.classList.remove('open'));
         });
         container.appendChild(btn);
     });