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/components/site-doc/SiteDocBreadcrumb.tsx

@@ -1,43 +0,0 @@
-import Link from "next/link";
-import { ChevronRight } from "lucide-react";
-
-export type Crumb = { href?: string; label: string };
-
-/** 位置指引：嵌在正文面板顶部，不单独成块 */
-export default function SiteDocBreadcrumb({
-  items,
-  className,
-}: {
-  items: Crumb[];
-  className?: string;
-}) {
-  return (
-    <nav
-      className={`j13-page-crumb meta flex items-center gap-1 flex-wrap text-[12.5px] leading-none ${className ?? ""}`}
-      aria-label="面包屑"
-    >
-      {items.map((item, i) => {
-        const last = i === items.length - 1;
-        const cls = last ? "truncate" : "shrink-0";
-        return (
-          <span key={`${item.label}-${i}`} className="inline-flex items-center gap-1 min-w-0">
-            {i > 0 ? <ChevronRight size={12} className="shrink-0 opacity-70" aria-hidden /> : null}
-            {item.href ? (
-              <Link
-                href={item.href}
-                className={`hover:text-[var(--accent)] transition-colors ${cls}`}
-                style={last ? { color: "var(--accent)" } : undefined}
-              >
-                {item.label}
-              </Link>
-            ) : (
-              <span className={cls} style={last ? { color: "var(--accent)" } : undefined}>
-                {item.label}
-              </span>
-            )}
-          </span>
-        );
-      })}
-    </nav>
-  );
-}

frontend/components/site-doc/SiteDocShell.tsx

@@ -8,14 +8,11 @@ export default function SiteDocShell({
   left,
   right,
   children,
-  breadcrumb,
   mobileExtras,
 }: {
   left?: ReactNode;
   right?: ReactNode;
   children: ReactNode;
-  /** 中栏正文上方的位置指引（流式排布，不再绝对定位） */
-  breadcrumb?: ReactNode;
   /** 窄屏下沉到正文之后的侧栏副本 */
   mobileExtras?: ReactNode;
 }) {
@@ -30,7 +27,6 @@ export default function SiteDocShell({
         </aside>
 
         <div className="min-w-0">
-          {breadcrumb}
           {children}
         </div>
 

frontend/components/site-doc/index.ts

@@ -4,6 +4,4 @@ export { default as AboutSiteHeader } from "./AboutSiteHeader";
 export { default as SiteDocPageNav } from "./SiteDocPageNav";
 export { default as SiteDocAnnoNav } from "./SiteDocAnnoNav";
 export { default as SiteDocMasthead } from "./SiteDocMasthead";
-export { default as SiteDocBreadcrumb } from "./SiteDocBreadcrumb";
-export type { Crumb } from "./SiteDocBreadcrumb";
 export type { SiteDocIdentityKind } from "./SiteDocIdentity";