fix(workspace): make Backspace merge work for untyped blocks and blocks with children

The merge only applied to a fixed list of block types, so legacy blocks
without a type were ignored, and blocks with children were rejected.
Now only code/divider/table/progress are excluded and children move up
under the previous block. The focused bullet dot is also neutral now.

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

frontend/src/features/workspace/OutlinerBlock.tsx

@@ -598,7 +598,7 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
               block.collapsed && hasChildren
                 ? 'bg-accent ring-2 ring-accent/30'
                 : activeFocus
-                ? 'bg-accent scale-125'
+                ? 'bg-main scale-125'
                 : 'bg-faint group-hover/bullet:bg-accent group-hover/bullet:scale-125'
             } ${
               isEmpty