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/core/entities/Post.ts

@@ -5,6 +5,8 @@
 
 // ==================== 帖子状态枚举 ====================
 
+import type { MessageSegment } from '../../types/dto/message';
+
 export type PostStatus = 'published' | 'draft' | 'deleted';
 
 // ==================== 值对象 ====================
@@ -73,6 +75,8 @@ export interface Post {
   isFavorited: boolean;
   /** 是否置顶 (camelCase) */
   isPinned: boolean;
+  /** 帖子内容段落（图文混排） */
+  segments?: MessageSegment[];
   /** 帖子状态 */
   status: PostStatus;
   /** 所属频道ID */