feat: add quote post selection state and preview card

app/(main)/post/new/page.tsx

@@ -109,7 +109,7 @@ export default function NewPostPage() {
             {postType === "quote" && (
               <div className="space-y-2">
                 <Label>引用帖子</Label>
-                <Select>
+                <Select value={selectedQuotePostId} onValueChange={setSelectedQuotePostId}>
                   <SelectTrigger>
                     <SelectValue placeholder="选择要引用的帖子" />
                   </SelectTrigger>
@@ -123,7 +123,27 @@ export default function NewPostPage() {
                 </Select>
                 <div className="mt-2 rounded-md border bg-muted/50 p-3 text-sm text-muted-foreground">
                   <p className="font-medium text-foreground">预览：</p>
-                  <p className="mt-1">选择一个帖子以查看预览...</p>
+                  {selectedQuotePostId ? (
+                    (() => {
+                      const post = mockPosts.find((p) => p.id === selectedQuotePostId)
+                      if (!post) return <p className="mt-1">未找到帖子</p>
+                      return (
+                        <div className="mt-2 rounded border bg-background p-3">
+                          <p className="font-medium text-foreground">{post.title}</p>
+                          <p className="mt-1 line-clamp-2 text-xs text-muted-foreground">
+                            {post.content}
+                          </p>
+                          <div className="mt-2 flex items-center gap-2 text-xs text-muted-foreground">
+                            <span>@{post.author.nickname}</span>
+                            <span>·</span>
+                            <span>{new Date(post.createdAt).toLocaleDateString("zh-CN")}</span>
+                          </div>
+                        </div>
+                      )
+                    })()
+                  ) : (
+                    <p className="mt-1">选择一个帖子以查看预览...</p>
+                  )}
                 </div>
               </div>
             )}