feat(ui): add ShareSheet component for multi-channel post sharing

Add ShareSheet component that provides native sharing options (WeChat, Moments, copy link, etc.) replacing the previous clipboard-only approach. Also change like icon from heart to thumb-up to better match the share functionality.

src/components/business/PostCard/PostCard.tsx

@@ -280,10 +280,10 @@ function createPostCardStyles(colors: AppColors) {
       marginLeft: 4,
     },
     activeActionText: {
-      color: colors.error.main,
+      color: colors.primary.main,
     },
     activeActionTextMerged: {
-      color: colors.error.main,
+      color: colors.primary.main,
       fontSize: fontSizes.sm,
       marginLeft: 4,
     },
@@ -589,7 +589,7 @@ const PostCardInner: React.FC<PostCardProps> = (normalizedProps) => {
             <Text style={styles.gridUsername} numberOfLines={1}>{author?.nickname || '匿名用户'}</Text>
           </TouchableOpacity>
           <View style={styles.gridLikeArea}>
-            <MaterialCommunityIcons name="heart-outline" size={14} color={colors.text.secondary} />
+            <MaterialCommunityIcons name="thumb-up-outline" size={14} color={colors.text.secondary} />
             <Text style={styles.gridLikeCount}>{formatNumber(post.likes_count || 0)}</Text>
           </View>
         </View>
@@ -677,9 +677,9 @@ const PostCardInner: React.FC<PostCardProps> = (normalizedProps) => {
         <View style={styles.actionButtons}>
           <TouchableOpacity style={styles.actionBtn} onPress={handleLike}>
             <MaterialCommunityIcons
-              name={post.is_liked ? 'heart' : 'heart-outline'}
+              name={post.is_liked ? 'thumb-up' : 'thumb-up-outline'}
               size={18}
-              color={post.is_liked ? colors.error.main : colors.text.secondary}
+              color={post.is_liked ? colors.primary.main : colors.text.secondary}
             />
             <Text style={post.is_liked ? styles.activeActionTextMerged : styles.actionText}>
               {post.likes_count > 0 ? formatNumber(post.likes_count) : '赞'}

src/components/business/ShareSheet/ShareSheet.tsx

@@ -0,0 +1,55 @@
+import React, { useEffect, useMemo } from 'react';
+import { Share as RNShare, Platform } from 'react-native';
+
+export interface ShareSheetProps {
+  visible: boolean;
+  postUrl?: string;
+  postTitle?: string;
+  onClose: () => void;
+  onShareAction?: (actionKey: string) => void;
+}
+
+const ShareSheet: React.FC<ShareSheetProps> = ({
+  visible,
+  postUrl,
+  postTitle,
+  onClose,
+  onShareAction,
+}) => {
+
+  const shareText = useMemo(() => {
+    const title = postTitle || '胡萝卜BBS帖子';
+    return `${title} ${postUrl || ''}`;
+  }, [postTitle, postUrl]);
+
+  useEffect(() => {
+    if (!visible) return;
+
+    const doShare = async () => {
+      try {
+        await RNShare.share(
+          {
+            message: shareText,
+            url: Platform.OS === 'ios' ? postUrl : undefined,
+            title: postTitle || '分享帖子',
+          },
+          {
+            subject: postTitle,
+            dialogTitle: '分享到',
+          }
+        );
+        onShareAction?.('system_share');
+      } catch {
+        // User cancelled
+      } finally {
+        onClose();
+      }
+    };
+
+    doShare();
+  }, [visible, shareText, postUrl, postTitle, onShareAction, onClose]);
+
+  return null;
+};
+
+export default ShareSheet;

src/components/business/ShareSheet/index.ts

@@ -0,0 +1,2 @@
+export { default } from './ShareSheet';
+export type { ShareSheetProps } from './ShareSheet';

src/components/business/index.ts

@@ -23,3 +23,4 @@ export { default as VoteCard } from './VoteCard';
 export { default as VoteEditor } from './VoteEditor';
 export { default as VotePreview } from './VotePreview';
 export { default as ReportDialog } from './ReportDialog';
+export { default as ShareSheet } from './ShareSheet';