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/app/settings/PasswordForm.tsx

@@ -0,0 +1,106 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import { apiChangePassword } from "@/lib/api";
+
+/**
+ * 修改密码表单：提交成功后后端已递增 TokenVersion 并清除全部登录 cookie，
+ * 刷新 layout 使 Header 回到游客态，随后跳转登录页重新登录。
+ */
+export default function PasswordForm() {
+  const router = useRouter();
+  const [oldPassword, setOldPassword] = useState("");
+  const [newPassword, setNewPassword] = useState("");
+  const [confirmPassword, setConfirmPassword] = useState("");
+  const [submitting, setSubmitting] = useState(false);
+  const [error, setError] = useState("");
+  const [success, setSuccess] = useState(false);
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+    setError("");
+
+    if (newPassword.length < 6) {
+      setError("新密码至少 6 位");
+      return;
+    }
+    if (newPassword !== confirmPassword) {
+      setError("两次输入的新密码不一致");
+      return;
+    }
+
+    setSubmitting(true);
+    try {
+      const res = await apiChangePassword(oldPassword, newPassword);
+      if (res.message) {
+        setSuccess(true);
+        // 后端已使全部 token 失效并清 cookie，刷新 layout 让 Header 回到游客态
+        router.refresh();
+        setTimeout(() => router.push("/login"), 1500);
+      } else {
+        setError(res.error || "修改失败");
+      }
+    } catch {
+      setError("网络错误，请重试");
+    } finally {
+      setSubmitting(false);
+    }
+  };
+
+  return (
+    <>
+      <p className="font-bold text-[15px] mb-3.5" style={{ color: "var(--ink)" }}>修改密码</p>
+
+      <form onSubmit={handleSubmit} className="panel p-5 sm:p-6 space-y-4">
+        {success && <div className="alert-ok" role="status">密码修改成功，正在跳转登录页...</div>}
+        {error && <div className="alert-error" role="alert">{error}</div>}
+        <div>
+          <label className="field-label" htmlFor="pwd-old">当前密码</label>
+          <input
+            id="pwd-old"
+            type="password"
+            value={oldPassword}
+            onChange={(e) => setOldPassword(e.target.value)}
+            required
+            autoComplete="current-password"
+            className="field"
+          />
+        </div>
+        <div>
+          <label className="field-label" htmlFor="pwd-new">新密码（至少 6 位）</label>
+          <input
+            id="pwd-new"
+            type="password"
+            value={newPassword}
+            onChange={(e) => setNewPassword(e.target.value)}
+            required
+            minLength={6}
+            autoComplete="new-password"
+            className="field"
+          />
+        </div>
+        <div>
+          <label className="field-label" htmlFor="pwd-confirm">确认新密码</label>
+          <input
+            id="pwd-confirm"
+            type="password"
+            value={confirmPassword}
+            onChange={(e) => setConfirmPassword(e.target.value)}
+            required
+            minLength={6}
+            autoComplete="new-password"
+            className="field"
+          />
+        </div>
+        <button
+          type="submit"
+          disabled={submitting || success}
+          className="btn btn-primary w-full justify-center mt-2"
+        >
+          {submitting ? "提交中..." : "修改密码"}
+        </button>
+      </form>
+    </>
+  );
+}