feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

frontend/app/admin/media/MediaLibrary.tsx

@@ -0,0 +1,274 @@
+"use client";
+
+import { useMemo, useState } from "react";
+import { Images, Copy, ExternalLink, Eye, Check } from "lucide-react";
+import Modal from "@/components/Modal";
+import AdminEmpty from "@/components/admin/AdminEmpty";
+import type { MediaLibraryItem } from "@/lib/api";
+
+function formatBytes(n: number): string {
+  if (n < 1024) return `${n} B`;
+  if (n < 1024 * 1024) return `${(n / 1024).toFixed(1)} KB`;
+  return `${(n / 1024 / 1024).toFixed(2)} MB`;
+}
+
+function formatTime(iso?: string): string {
+  if (!iso) return "—";
+  const d = new Date(iso);
+  if (Number.isNaN(d.getTime())) return "—";
+  return d.toLocaleString("zh-CN", { hour12: false });
+}
+
+function formatDims(item: MediaLibraryItem): string {
+  if (item.width > 0 && item.height > 0) return `${item.width} × ${item.height}`;
+  return "—";
+}
+
+/** 网格缩略图：本地图走服务端缩略接口（磁盘缓存），远程对象直连原图（头像类本身小） */
+function thumbSrc(item: MediaLibraryItem): string {
+  if (item.url.startsWith("/uploads/")) {
+    return `/api/admin/media-library/thumb?u=${encodeURIComponent(item.url)}`;
+  }
+  return item.url;
+}
+
+/** 管理后台媒体库：全站图片网格 + 分类筛选 + 详情弹窗（WebP 优先，可查看原图） */
+export default function MediaLibrary({
+  items,
+  counts,
+  total,
+  initError,
+}: {
+  items: MediaLibraryItem[];
+  counts: Record<string, number>;
+  total: number;
+  initError?: string;
+}) {
+  const [category, setCategory] = useState<string>("all");
+  const [active, setActive] = useState<MediaLibraryItem | null>(null);
+  const [showOriginal, setShowOriginal] = useState(false);
+  const [copied, setCopied] = useState(false);
+
+  const filtered = useMemo(
+    () => (category === "all" ? items : items.filter((i) => i.category === category)),
+    [items, category]
+  );
+
+  const tabs = useMemo(() => {
+    const list: { key: string; label: string; count: number }[] = [
+      { key: "all", label: "全部", count: total },
+    ];
+    // 按现有条目实际分类生成 tab，保持稳定顺序
+    const seen = new Map<string, string>();
+    for (const i of items) seen.set(i.category, i.category_name);
+    for (const [key, label] of seen) {
+      list.push({ key, label, count: counts[key] ?? 0 });
+    }
+    return list;
+  }, [items, counts, total]);
+
+  const openItem = (item: MediaLibraryItem) => {
+    setActive(item);
+    setShowOriginal(false);
+    setCopied(false);
+  };
+
+  const copyLink = async (item: MediaLibraryItem) => {
+    try {
+      await navigator.clipboard.writeText(`${window.location.origin}${item.url}`);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 1500);
+    } catch {
+      /* 剪贴板不可用静默 */
+    }
+  };
+
+  if (initError) {
+    return (
+      <AdminEmpty
+        icon={<Images size={20} />}
+        title="媒体库加载失败"
+        description={initError}
+      />
+    );
+  }
+
+  if (total === 0) {
+    return (
+      <AdminEmpty
+        icon={<Images size={20} />}
+        title="暂无图片资源"
+        description="站点内还没有任何上传图片"
+      />
+    );
+  }
+
+  return (
+    <div>
+      {/* 分类 tab */}
+      <div className="flex items-center gap-1.5 flex-wrap mb-4">
+        {tabs.map((t) => {
+          const on = category === t.key;
+          return (
+            <button
+              key={t.key}
+              type="button"
+              onClick={() => setCategory(t.key)}
+              aria-pressed={on}
+              className="h-8 px-3 rounded-full text-[12.5px] font-medium inline-flex items-center gap-1.5 transition-colors"
+              style={{
+                border: `1px solid ${on ? "var(--accent)" : "var(--line)"}`,
+                background: on ? "var(--accent-soft)" : "var(--panel)",
+                color: on ? "var(--accent)" : "var(--ink-2)",
+              }}
+            >
+              {t.label}
+              <span className="tabular-nums opacity-70">{t.count}</span>
+            </button>
+          );
+        })}
+      </div>
+
+      {/* 图片网格 */}
+      <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 xl:grid-cols-6 gap-3">
+        {filtered.map((item) => (
+          <button
+            key={item.url}
+            type="button"
+            onClick={() => openItem(item)}
+            className="group text-left rounded-xl overflow-hidden transition-transform hover:-translate-y-0.5 focus-visible:-translate-y-0.5"
+            style={{ background: "var(--panel)", border: "1px solid var(--line)" }}
+            title={item.name}
+          >
+            <span className="block relative aspect-square overflow-hidden">
+              <img
+                src={thumbSrc(item)}
+                alt={item.name}
+                loading="lazy"
+                className="absolute inset-0 w-full h-full object-cover"
+              />
+              {item.original_url && (
+                <span
+                  className="absolute top-1.5 left-1.5 px-1.5 py-0.5 rounded text-[10px] font-semibold"
+                  style={{ background: "var(--accent-soft)", color: "var(--accent)" }}
+                >
+                  WebP
+                </span>
+              )}
+            </span>
+            <span className="block px-2.5 py-2">
+              <span
+                className="block text-[12px] font-medium truncate"
+                style={{ color: "var(--ink)" }}
+              >
+                {item.name}
+              </span>
+              <span className="block meta text-[11px] mt-0.5">
+                {formatDims(item)} · {formatBytes(item.size)}
+              </span>
+            </span>
+          </button>
+        ))}
+      </div>
+
+      {/* 详情弹窗 */}
+      <Modal
+        open={active !== null}
+        onClose={() => setActive(null)}
+        title={active?.name ?? ""}
+        description={active ? `${active.category_name}${active.uploader ? ` · 上传者 ${active.uploader}` : ""}` : undefined}
+        maxWidthClass="max-w-2xl"
+        dismissOnOverlay
+        bodyClassName=""
+      >
+        {active && (
+          <div>
+            <div
+              className="rounded-xl overflow-hidden flex items-center justify-center"
+              style={{
+                background:
+                  "repeating-conic-gradient(var(--panel-2) 0% 25%, var(--panel) 0% 50%) 50% / 20px 20px",
+                maxHeight: "52vh",
+              }}
+            >
+              <img
+                src={showOriginal && active.original_url ? active.original_url : active.url}
+                alt={active.name}
+                className="max-w-full max-h-[52vh] w-auto h-auto object-contain"
+              />
+            </div>
+
+            {active.original_url && (
+              <p className="meta text-[12px] mt-2.5">
+                {showOriginal
+                  ? "当前显示原图（原格式，体积更大）"
+                  : "当前显示 WebP 压缩版；站点内容引用的均为 WebP"}
+              </p>
+            )}
+
+            <dl className="mt-4 grid grid-cols-2 gap-x-4 gap-y-2 text-[12.5px]">
+              {[
+                ["分类", active.category_name],
+                ["尺寸", formatDims(active)],
+                ["大小", formatBytes(active.size)],
+                ["格式", active.mime],
+                ["上传者", active.uploader || "—"],
+                ["时间", formatTime(active.uploaded_at)],
+              ].map(([k, v]) => (
+                <div key={k} className="flex items-baseline gap-2 min-w-0">
+                  <dt className="meta shrink-0 w-11 text-right">{k}</dt>
+                  <dd className="truncate" style={{ color: "var(--ink)" }} title={v}>
+                    {v}
+                  </dd>
+                </div>
+              ))}
+              {active.source_url && (
+                <div className="flex items-baseline gap-2 min-w-0 col-span-2">
+                  <dt className="meta shrink-0 w-11 text-right">来源</dt>
+                  <dd className="truncate">
+                    <a
+                      href={active.source_url}
+                      target="_blank"
+                      rel="noreferrer"
+                      title={active.source_label || active.source_url}
+                      className="hover:underline"
+                      style={{ color: "var(--accent)" }}
+                    >
+                      {active.source_label || active.source_url}
+                    </a>
+                  </dd>
+                </div>
+              )}
+            </dl>
+
+            <div className="mt-5 flex items-center justify-end gap-2 flex-wrap">
+              {active.original_url && (
+                <button
+                  type="button"
+                  className="btn btn-sm"
+                  onClick={() => setShowOriginal((v) => !v)}
+                >
+                  <Eye size={14} aria-hidden />
+                  {showOriginal ? "看 WebP" : "查看原图"}
+                </button>
+              )}
+              <button type="button" className="btn btn-sm" onClick={() => copyLink(active)}>
+                {copied ? <Check size={14} aria-hidden /> : <Copy size={14} aria-hidden />}
+                {copied ? "已复制" : "复制链接"}
+              </button>
+              <a
+                className="btn btn-sm"
+                href={active.original_url && showOriginal ? active.original_url : active.url}
+                target="_blank"
+                rel="noreferrer"
+              >
+                <ExternalLink size={14} aria-hidden />
+                新窗口打开
+              </a>
+            </div>
+          </div>
+        )}
+      </Modal>
+    </div>
+  );
+}