Introduce a new `BlockEditor` component and upgrade the post/message
rendering system to support rich text segments (images, @mentions,
votes, and post references).
- Implement `BlockEditor` for long-form post creation with image
embedding support.
- Upgrade `PostContentRenderer` and `SegmentRenderer` to handle
complex segment types including inline images and block elements.
- Refactor `PostCard` to use segment-based content and image
signatures for optimized memoization.
- Centralize segment partitioning logic in `segmentUtils.ts` to ensure
consistent rendering across chat and post modules.
- Update `PostRepository` and `Post` entity to support the new
`segments` data structure.
