feat: keep placing mode active for multiple placements

Selected gate type stays active after placing, allowing multiple
gates of the same type without re-selecting. Right-click or Escape
to cancel placing mode.

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

js/events.js

@@ -90,7 +90,7 @@ export function initEvents() {
             }
             state.gates.push(newGate);
             evaluateAll();
-            state.placingGate = null;
+            // Keep placingGate active so user can place multiple — right-click to cancel
             return;
         }
 
@@ -172,6 +172,13 @@ export function initEvents() {
 
     canvas.addEventListener('contextmenu', e => {
         e.preventDefault();
+
+        // Right-click cancels placing mode
+        if (state.placingGate) {
+            state.placingGate = null;
+            return;
+        }
+
         const world = screenToWorld(e.offsetX, e.offsetY);
         const port = findPortAt(world.x, world.y);
         if (port && port.type === 'input') {