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

backend/handler/upload.go

@@ -3,8 +3,11 @@ package handler
 import (
 	"errors"
 	"io"
+	"mime"
 	"net/http"
+	"path"
 	"strconv"
+	"time"
 
 	"github.com/freefire/jiang13-bbs/middleware"
 	"github.com/freefire/jiang13-bbs/service"
@@ -311,6 +314,75 @@ func (h *Handlers) UploadBrandFromMedia(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"url": url})
 }
 
+// UploadLibraryCoverFromMedia 把本人媒体库图片复制一份作书籍封面素材，只返回 URL，不绑定条目
+func (h *Handlers) UploadLibraryCoverFromMedia(c *gin.Context) {
+	var req struct {
+		AttachmentID uint `json:"attachment_id"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil || req.AttachmentID == 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "请选择媒体库中的图片"})
+		return
+	}
+	claims := middleware.CurrentUser(c)
+	if claims == nil {
+		c.JSON(http.StatusUnauthorized, gin.H{"error": "请先登录"})
+		return
+	}
+	att, err := h.Upload.CopyImageFromMedia(claims.ID, req.AttachmentID)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"url": att.URL, "attachment": att})
+}
+
+// brandAliasSlots 固定地址路径段 → 品牌槽位
+var brandAliasSlots = map[string]string{
+	"logo-light": service.BrandSlotLight,
+	"logo-dark":  service.BrandSlotDark,
+	"favicon":    service.BrandSlotFavicon,
+}
+
+// BrandImage 品牌图固定对外地址（GET /api/brand/logo-light|logo-dark|favicon）：
+// 友链等外部引用此地址，站点换图后按设置解析到当前图，地址永不变。
+func (h *Handlers) BrandImage(c *gin.Context) {
+	slot, ok := brandAliasSlots[c.Param("slot")]
+	if !ok {
+		c.Status(http.StatusNotFound)
+		return
+	}
+	var url string
+	var err error
+	switch slot {
+	case service.BrandSlotLight:
+		url, err = h.Setting.LogoLightURL()
+	case service.BrandSlotDark:
+		url, err = h.Setting.LogoDarkURL()
+	default:
+		url, err = h.Setting.FaviconURL()
+	}
+	if err != nil {
+		c.Status(http.StatusInternalServerError)
+		return
+	}
+	if url == "" {
+		c.Status(http.StatusNotFound)
+		return
+	}
+	f, err := h.Upload.OpenBrandFile(url)
+	if err != nil {
+		c.Status(http.StatusNotFound)
+		return
+	}
+	defer f.Close()
+	// 扩展名定 Content-Type（SVG 不能靠嗅探）；短缓存让换图在友链侧尽快生效
+	if ct := mime.TypeByExtension(path.Ext(url)); ct != "" {
+		c.Header("Content-Type", ct)
+	}
+	c.Header("Cache-Control", "public, max-age=3600")
+	http.ServeContent(c.Writer, c.Request, path.Base(url), time.Time{}, f)
+}
+
 // AdminMediaLibrary 管理后台媒体库：全站图片盘点（磁盘五类目录 + 附件元数据）
 func (h *Handlers) AdminMediaLibrary(c *gin.Context) {
 	items, counts, err := h.Upload.AdminMediaLibrary()