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/app/admin/library/LibraryCoverPicker.tsx

@@ -0,0 +1,97 @@
+"use client";
+
+import { Images, Loader2 } from "lucide-react";
+import { useEffect, useState } from "react";
+import Modal from "@/components/Modal";
+import { apiMyMedia, type MediaAttachment } from "@/lib/api";
+
+/** 书籍封面图库选择弹窗：列出本人媒体库图片，选中后由父组件复制一份作封面素材 */
+export default function LibraryCoverPicker({
+  open,
+  onClose,
+  onPick,
+}: {
+  open: boolean;
+  onClose: () => void;
+  onPick: (item: MediaAttachment) => void;
+}) {
+  const [items, setItems] = useState<MediaAttachment[]>([]);
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState("");
+
+  useEffect(() => {
+    if (!open) return;
+    let cancelled = false;
+    setLoading(true);
+    setError("");
+    void apiMyMedia()
+      .then((res) => {
+        if (cancelled) return;
+        if (res.error) {
+          setError(res.error);
+          setItems([]);
+          return;
+        }
+        setItems((res.attachments || []).filter((item) => item.kind === "image"));
+      })
+      .catch(() => {
+        if (!cancelled) setError("加载媒体库失败");
+      })
+      .finally(() => {
+        if (!cancelled) setLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [open]);
+
+  return (
+    <Modal
+      open={open}
+      title="从图库选择封面"
+      description="选择后会复制一份作为封面素材，之后删除原图不影响封面。"
+      onClose={onClose}
+      maxWidthClass="max-w-lg"
+      dismissOnOverlay
+    >
+      {loading ? (
+        <p className="meta text-[13px] flex items-center gap-2 py-6 justify-center">
+          <Loader2 size={14} className="animate-spin" /> 加载中
+        </p>
+      ) : error ? (
+        <p className="text-[13px] py-4" style={{ color: "var(--danger, #b42318)" }}>
+          {error}
+        </p>
+      ) : items.length === 0 ? (
+        <p className="meta text-[13px] py-6 text-center">
+          图库还没有图片。上传封面或发帖插图后，会自动出现在这里。
+        </p>
+      ) : (
+        <div className="grid grid-cols-4 sm:grid-cols-5 gap-2 max-h-[46vh] overflow-y-auto">
+          {items.map((item) => (
+            <button
+              key={item.id}
+              type="button"
+              onClick={() => {
+                onPick(item);
+                onClose();
+              }}
+              className="relative group aspect-square rounded-xl overflow-hidden"
+              style={{ boxShadow: "0 0 0 1px var(--line)", background: "var(--panel-2)" }}
+              data-tip="复制一份用作封面"
+            >
+              {/* eslint-disable-next-line @next/next/no-img-element */}
+              <img src={item.url} alt="图库候选封面" className="w-full h-full object-contain" loading="lazy" />
+              <span
+                className="pointer-events-none absolute inset-0 flex items-center justify-center opacity-0 group-hover:opacity-100"
+                style={{ background: "color-mix(in srgb, var(--ink) 40%, transparent)", color: "#fff" }}
+              >
+                <Images size={14} />
+              </span>
+            </button>
+          ))}
+        </div>
+      )}
+    </Modal>
+  );
+}