Fix merge queue delete: remove from neuroglancer DOM, not just store

The merge queue is scraped from neuroglancer's DOM via MutationObserver,
so splicing our store array was immediately overwritten. Now clicks the
neuroglancer delete button or removes the DOM node directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: amyleesterling

src/store.ts

@@ -1651,6 +1651,27 @@ export const useSplitMergeOverlayStore = defineStore('splitMergeOverlay', () =>
   }
 
   function removeMergeSegment(index: number) {
+    // Remove from neuroglancer's DOM (the source of truth)
+    try {
+      const mergeEl = document.querySelector('.graphene-merge-segments');
+      if (mergeEl) {
+        const submissions = mergeEl.querySelectorAll('.graphene-merge-segments-submission');
+        const sub = submissions[index];
+        if (sub) {
+          // Look for a delete/close/remove button within the submission
+          const deleteBtn = sub.querySelector('button[title*="delete" i], button[title*="remove" i], button[title*="cancel" i], .neuroglancer-icon[title*="delete" i]') as HTMLElement | null;
+          if (deleteBtn) {
+            deleteBtn.click();
+          } else {
+            // No button found — try removing the DOM node directly
+            sub.remove();
+          }
+        }
+      }
+    } catch (e) {
+      console.warn('[smo] removeMergeSegment DOM error:', e);
+    }
+    // Also update local state immediately for UI responsiveness
     mergeSegments.value.splice(index, 1);
     if (mergeSubmissionCount.value > 0) mergeSubmissionCount.value--;
   }