feat(business): implement block-based content editing and rich text rendering

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.

src/components/business/index.ts

@@ -24,6 +24,8 @@ export { default as VoteEditor } from './VoteEditor';
 export { default as VotePreview } from './VotePreview';
 export { default as PostContentRenderer } from './PostContentRenderer';
 export { default as PostMentionInput } from './PostMentionInput';
+export { default as BlockEditor } from './BlockEditor';
+export type { BlockEditorHandle } from './BlockEditor';
 export { default as ReportDialog } from './ReportDialog';
 export { default as ShareSheet } from './ShareSheet';
 export { TradeCard } from './TradeCard/TradeCard';