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/markdown/MdTimeline.tsx

@@ -48,34 +48,44 @@ export default function MdTimeline({ items, renderBody }: MdTimelineProps) {
           style={{ scrollMarginTop: 90 }}
           aria-labelledby={`${timelineDayId(g.date)}-label`}
         >
-          <div className="j13-timeline-day-label" id={`${timelineDayId(g.date)}-label`}>
-            <time dateTime={/^\d{4}-\d{2}-\d{2}$/.test(g.date) ? g.date : undefined}>
-              {g.date}
-            </time>
-            <span className="j13-timeline-day-count">{g.items.length}</span>
-          </div>
           <ul className="j13-timeline-items">
-            {g.items.map((it) => {
-              const idx = flat++;
+            {g.items.map((it, idx) => {
+              const flatIdx = flat++;
               const { kind, scope, rest } = splitTitlePrefix(it.title);
               const bodyNode = renderBody
-                ? renderBody(it, idx)
+                ? renderBody(it, flatIdx)
                 : it.body.trim()
                   ? <span className="whitespace-pre-wrap">{it.body}</span>
                   : null;
+              const isFirstInDay = idx === 0;
 
               return (
-                <li key={`${g.date}-${idx}-${it.title}`} className="j13-timeline-item" role="listitem">
+                <li
+                  key={`${g.date}-${flatIdx}-${it.title}`}
+                  className={`j13-timeline-item${isFirstInDay ? " is-first-in-day" : ""}`}
+                  role="listitem"
+                >
                   <span className="j13-timeline-dot" aria-hidden />
                   <div className="j13-timeline-card">
                     <div className="j13-timeline-head">
+                      {isFirstInDay ? (
+                        <div
+                          className="j13-timeline-day-label"
+                          id={`${timelineDayId(g.date)}-label`}
+                        >
+                          <time dateTime={/^\d{4}-\d{2}-\d{2}$/.test(g.date) ? g.date : undefined}>
+                            {g.date}
+                          </time>
+                          <span className="j13-timeline-day-count">{g.items.length}</span>
+                        </div>
+                      ) : null}
+                      <p className="j13-timeline-summary">{rest}</p>
                       {kind ? (
                         <span className={`j13-timeline-chip is-${kind}`}>
                           {kind}
                           {scope ? <span className="j13-timeline-chip-scope">({scope})</span> : null}
                         </span>
                       ) : null}
-                      <p className="j13-timeline-summary">{rest}</p>
                       {it.sourceUrl ? (
                         <a
                           href={it.sourceUrl}