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/__tests__/OutlinerBlock.caret.test.tsx

@@ -207,6 +207,33 @@ describe('slash command menu and outliner fixes', () => {
     expect(onCreateBelow).toHaveBeenCalledWith('b1', '');
   });
 
+  it('keeps an empty indented bullet on its level when another bullet on that level follows', () => {
+    const onIndent = vi.fn();
+    const onCreateBelow = vi.fn();
+    const { container } = render(
+      <OutlinerBlock
+        docId="d"
+        block={{ id: 'b1', type: 'bullet', content: '', level: 1 } as DocumentBlock}
+        index={0}
+        isLast={false}
+        siblingFollows
+        maxAllowedLevel={2}
+        hasChildren={false}
+        isFocused
+        canEdit
+        onUpdateBlock={() => {}}
+        onCreateBlockBelow={onCreateBelow}
+        onDeleteBlock={() => {}}
+        onIndentBlock={onIndent}
+        onNavigateFocus={() => {}}
+        registerTextareaRef={() => {}}
+      />
+    );
+    fireEvent.keyDown(container.querySelector('textarea') as HTMLTextAreaElement, { key: 'Enter' });
+    expect(onIndent).not.toHaveBeenCalled();
+    expect(onCreateBelow).toHaveBeenCalledWith('b1', '');
+  });
+
   it('inserts a bare URL for /link with a web address in an empty bullet, and [Text](url) otherwise', () => {
     const updates: DocumentBlock[] = [];
     const block = { id: 'b1', type: 'bullet', content: '', level: 0 } as DocumentBlock;