feat: 新增站点公告系统与多项体验优化

此提交完成了以下核心功能与优化：
1. 新增站点公告全流程支持：包含公告模型、CRUD接口、管理后台页面、公告详情页与首页展示
2. 实现评论楼层化与回复嵌套：重构评论系统为树状结构，支持回复评论与层级限制
3. 添加用户在线状态追踪：异步更新用户最后活跃时间并做SQL限频
4. 优化用户体验：移动端浮动操作条、统一板块标签样式、用户页签到积分展示、注册即登录优化
5. 其他细节优化：SEO站点地图新增公告页、评论分页与统计、登录/注册表单增强等

frontend/components/MobilePostBar.tsx

@@ -0,0 +1,82 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { ArrowUp, MessageSquare } from "lucide-react";
+import LikeButton from "./LikeButton";
+
+/**
+ * 帖子详情移动端浮动操作条（lg 以下显示）：
+ * 长文读到中途也能点赞 / 跳评论框 / 回顶部，不必滚到文末。
+ */
+export default function MobilePostBar({
+  postId,
+  liked,
+  likeCount,
+}: {
+  postId: string;
+  liked: boolean;
+  likeCount: number;
+}) {
+  const [showTop, setShowTop] = useState(false);
+
+  useEffect(() => {
+    const onScroll = () => setShowTop(window.scrollY > 400);
+    onScroll();
+    window.addEventListener("scroll", onScroll, { passive: true });
+    return () => window.removeEventListener("scroll", onScroll);
+  }, []);
+
+  const scrollToComments = () => {
+    document.getElementById("comment-input")?.scrollIntoView({
+      behavior: window.matchMedia("(prefers-reduced-motion: reduce)").matches ? "auto" : "smooth",
+      block: "center",
+    });
+  };
+
+  const scrollToTop = () => {
+    window.scrollTo({
+      top: 0,
+      behavior: window.matchMedia("(prefers-reduced-motion: reduce)").matches ? "auto" : "smooth",
+    });
+  };
+
+  return (
+    <div className="lg:hidden fixed bottom-4 left-1/2 -translate-x-1/2 z-40" role="navigation" aria-label="阅读操作">
+      <div
+        className="flex items-center gap-0.5 rounded-full border px-1.5 py-1.5"
+        style={{
+          background: "color-mix(in srgb, var(--panel) 90%, transparent)",
+          borderColor: "var(--line-2)",
+          boxShadow: "var(--shadow-lg)",
+          backdropFilter: "blur(14px)",
+          WebkitBackdropFilter: "blur(14px)",
+        }}
+      >
+        <button
+          type="button"
+          onClick={scrollToComments}
+          className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--panel-2)]"
+          style={{ color: "var(--ink-2)" }}
+          aria-label="跳到评论框"
+        >
+          <MessageSquare size={18} />
+        </button>
+        {/* 放大点赞按钮的触控区域（样式见 globals.css .j13-bar-like） */}
+        <div className="j13-bar-like">
+          <LikeButton postId={postId} liked={liked} likeCount={likeCount} variant="compact" />
+        </div>
+        {showTop && (
+          <button
+            type="button"
+            onClick={scrollToTop}
+            className="w-11 h-11 flex items-center justify-center rounded-full transition-colors hover:bg-[var(--panel-2)] j13-toast-in"
+            style={{ color: "var(--ink-2)" }}
+            aria-label="回到顶部"
+          >
+            <ArrowUp size={18} />
+          </button>
+        )}
+      </div>
+    </div>
+  );
+}