fix(workspace): Enter keeps an empty bullet on its level when siblings follow

Outdenting an empty bullet on Enter only happens when no bullet on the
same level follows in the branch; otherwise a new bullet is added below.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/OutlinerTree.tsx

@@ -244,6 +244,21 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
 
   const listLabels = useMemo(() => numberedLabels(blocks), [blocks]);
 
+  // Whether another bullet on the same level follows (before the branch ends); an empty bullet
+  // is then not outdented by Enter
+  const siblingFollows = useCallback(
+    (index: number) => {
+      const level = blocks[index]?.level || 0;
+      for (let i = index + 1; i < blocks.length; i++) {
+        const l = blocks[i].level || 0;
+        if (l < level) return false;
+        if (l === level) return true;
+      }
+      return false;
+    },
+    [blocks]
+  );
+
   const handleCreateBlockBelow = useCallback(
     (afterBlockId: string, newContent: string = '') => {
       if (!canEdit) return;
@@ -617,6 +632,7 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
             block={block}
             index={index}
             isLast={isLast}
+            siblingFollows={siblingFollows(index)}
             maxAllowedLevel={maxAllowedLevel}
             hasChildren={hasChildren}
             isFocused={focusedBlockId === block.id}