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/BlockEditor/BlockEditor.tsx

@@ -0,0 +1,118 @@
+import React, { useImperativeHandle, useCallback } from 'react';
+import { View, ScrollView, StyleSheet } from 'react-native';
+
+import { useAppColors, spacing } from '../../../theme';
+import { MessageSegment } from '../../../types';
+import TextBlockInput from './TextBlockInput';
+import ImageBlockView from './ImageBlockView';
+import { useBlockEditor } from './useBlockEditor';
+
+export interface BlockEditorHandle {
+  insertImage: () => Promise<void>;
+  insertTextAtCursor: (text: string) => void;
+  getSegments: () => MessageSegment[];
+  getContent: () => string;
+  hasUploadingImages: () => boolean;
+  focus: () => void;
+}
+
+interface BlockEditorProps {
+  placeholder?: string;
+  maxLength?: number;
+  style?: any;
+  textStyle?: any;
+  onContentChange?: (content: string) => void;
+}
+
+const BlockEditor = React.forwardRef<BlockEditorHandle, BlockEditorProps>(
+  ({ placeholder = '添加正文', maxLength = 2000, style, textStyle, onContentChange }, ref) => {
+    const colors = useAppColors();
+    const editor = useBlockEditor();
+
+    useImperativeHandle(ref, () => ({
+      insertImage: editor.insertImage,
+      insertTextAtCursor: editor.insertTextAtCursor,
+      getSegments: editor.getSegments,
+      getContent: editor.getContent,
+      hasUploadingImages: editor.hasUploadingImages,
+      focus: () => {
+        const firstTextBlock = editor.blocks.find(b => b.type === 'text');
+        if (firstTextBlock) {
+          editor.focusBlock(firstTextBlock.id);
+        }
+      },
+    }));
+
+    const handleTextChange = useCallback(
+      (blockId: string, text: string, mentions: any) => {
+        editor.updateTextBlock(blockId, text, mentions);
+        onContentChange?.(editor.getContent());
+      },
+      [editor.updateTextBlock, editor.getContent, onContentChange],
+    );
+
+    const handleRemoveImage = useCallback(
+      (blockId: string) => {
+        editor.removeImageBlock(blockId);
+        onContentChange?.(editor.getContent());
+      },
+      [editor.removeImageBlock, editor.getContent, onContentChange],
+    );
+
+    return (
+      <View style={[styles.container, style]}>
+        <ScrollView
+          style={styles.scroll}
+          contentContainerStyle={styles.scrollContent}
+          showsVerticalScrollIndicator={false}
+          keyboardShouldPersistTaps="handled"
+          nestedScrollEnabled
+        >
+          {editor.blocks.map(block => {
+            if (block.type === 'text') {
+              return (
+                <TextBlockInput
+                  key={block.id}
+                  block={block}
+                  isActive={editor.activeBlockId === block.id}
+                  placeholder={placeholder}
+                  maxLength={maxLength}
+                  onTextChange={handleTextChange}
+                  onFocusBlock={editor.setActiveBlockId}
+                  onSelectMention={() => {}}
+                  inputRef={(ref) => editor.registerInputRef(block.id, ref)}
+                  style={textStyle}
+                />
+              );
+            }
+
+            return (
+              <ImageBlockView
+                key={block.id}
+                block={block}
+                onRemove={handleRemoveImage}
+              />
+            );
+          })}
+        </ScrollView>
+      </View>
+    );
+  },
+);
+
+BlockEditor.displayName = 'BlockEditor';
+
+const styles = StyleSheet.create({
+  container: {
+    flexGrow: 1,
+  },
+  scroll: {
+    flexGrow: 1,
+  },
+  scrollContent: {
+    flexGrow: 1,
+    paddingBottom: spacing.xs,
+  },
+});
+
+export default BlockEditor;