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/ShareSheet/ShareSheet.tsx

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+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

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+export { default } from './ShareSheet';
+export type { ShareSheetProps } from './ShareSheet';