fix: show post categories as tree

[Higurashi-kagome]

Summary

• render post categories in the post settings UI with parent-child tree indentation
• keep parent and child selection independent so existing multi-select behavior is preserved
• improve category readability for nested categories reported in 完善文章设置的子分类的显示 #334

Test plan

• Not tested locally yet because the project is still not debuggable in my environment