feat: add mention functionality to PostComposer and refactor post display in PostPage

- Implemented a mention dropdown in PostComposer.vue that allows users to mention other users by typing '@'.
- Added logic to fetch user suggestions based on the mention query and display them in a dropdown.
- Refactored the Post display in [id].vue to use PostCard component for a cleaner and more consistent UI.
- Removed unnecessary code related to media viewer and post actions from [id].vue.
- Updated styles to accommodate the new PostCard layout and mention dropdown.

Author: Sunwuyuan