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/screens/message/components/ChatScreen/styles.ts

@@ -296,11 +296,11 @@ export function createChatScreenStyles(colors: AppColors, dynamicStyles?: {
     alignItems: 'flex-start',
   },
   senderName: {
-    fontSize: 14,
-    color: textPrimary,
+    fontSize: 12,
+    color: textSecondary,
     marginBottom: 4,
     marginLeft: 4,
-    fontWeight: '600',
+    fontWeight: '500',
   },
   mySenderName: {
     marginLeft: 0,