feat: 友链/书库/移动端导航/排行榜等多模块功能

新增模块：
- 友链（friendlink）后端处理 + 前端管理页与友链板
- 书库（library）：导入、章节、封面、阅读页、横竖版自适应 AdaptiveCoverSlot
- 顶栏导航（header_nav）配置与 MobileTabBar/MobileRailDrawers 移动端抽屉
- 排行榜 service 测试、站点页面测试、时间线发布（timeline_release）

其它改动：
- 后端 handlers/services 全量小幅调整
- 前端组件、库函数、URL/品牌/站点 URL 工具更新
- Lightbox 图片、MdEntries 条目卡、coverColor 派生色相等前端能力

frontend/components/SiteBrandProvider.tsx

@@ -4,6 +4,8 @@ import { createContext, useContext, useEffect, useMemo, useRef, useState, type R
 import { apiGetSettings, type PublicSettings } from "@/lib/api";
 import { siteDocumentTitle, normalizeBrandLogoFit, normalizeBrandLogoSize, normalizeBrandMark, normalizeFooterLinks } from "@/lib/brand";
 import { normalizeUrlStyle, type UrlStyle } from "@/lib/urlStyle";
+import { DEFAULT_MOBILE_TABS, normalizeMobileTabs, type MobileTabKey } from "@/lib/mobileTabs";
+import { DEFAULT_HEADER_NAV, normalizeHeaderNav, type HeaderNavItem } from "@/lib/headerNav";
 import { realtime, RT_SETTINGS_CHANGED, type RtSettingsData } from "@/lib/realtime";
 import { onDocumentVisible, VISIBLE_RECONCILE_GAP_MS } from "@/lib/visibilityReconcile";
 
@@ -19,6 +21,8 @@ export type SiteBrand = {
   brand_logo_fit: PublicSettings["brand_logo_fit"];
   footer_links: PublicSettings["footer_links"];
   url_style: UrlStyle;
+  mobile_tabs: MobileTabKey[];
+  header_nav: HeaderNavItem[];
 };
 
 const SiteBrandContext = createContext<SiteBrand | null>(null);
@@ -36,6 +40,8 @@ function fromSettings(s: PublicSettings): SiteBrand {
     brand_logo_fit: s.brand_logo_fit,
     footer_links: s.footer_links,
     url_style: s.url_style,
+    mobile_tabs: s.mobile_tabs,
+    header_nav: s.header_nav,
   };
 }
 
@@ -51,7 +57,9 @@ function sameBrand(a: SiteBrand, b: SiteBrand): boolean {
     a.brand_logo_size === b.brand_logo_size &&
     a.brand_logo_fit === b.brand_logo_fit &&
     a.url_style === b.url_style &&
-    JSON.stringify(a.footer_links) === JSON.stringify(b.footer_links)
+    JSON.stringify(a.footer_links) === JSON.stringify(b.footer_links) &&
+    JSON.stringify(a.mobile_tabs) === JSON.stringify(b.mobile_tabs) &&
+    JSON.stringify(a.header_nav) === JSON.stringify(b.header_nav)
   );
 }
 
@@ -102,6 +110,8 @@ export function useSiteBrand(): SiteBrand {
       brand_logo_fit: "contain",
       footer_links: [],
       url_style: "default",
+      mobile_tabs: [...DEFAULT_MOBILE_TABS],
+      header_nav: DEFAULT_HEADER_NAV.map((item) => ({ ...item })),
     };
   }
   return ctx;
@@ -153,6 +163,8 @@ export default function SiteBrandProvider({
           ? normalizeFooterLinks(raw.footer_links)
           : prev.footer_links,
         url_style: raw.url_style !== undefined ? normalizeUrlStyle(raw.url_style) : prev.url_style,
+        mobile_tabs: raw.mobile_tabs !== undefined ? normalizeMobileTabs(raw.mobile_tabs) : prev.mobile_tabs,
+        header_nav: raw.header_nav !== undefined ? normalizeHeaderNav(raw.header_nav) : prev.header_nav,
       };
       if (sameBrand(prev, next)) return;
       retitle(prev, next);