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

backend/service/upload.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/freefire/jiang13-bbs/model"
+	webpenc "github.com/gen2brain/webp"
 	"golang.org/x/image/webp"
 	"gorm.io/gorm"
 )
@@ -29,10 +30,13 @@ const (
 	AvatarMinDim   = 64
 	AvatarMaxDim   = 512
 
-	// 帖子插图：允许 JPEG/PNG/WebP，不强制转码
-	ImageMaxBytes = 5 << 20 // 5 MiB
+	// 帖子插图：JPEG/PNG 统一转存 WebP；GIF（动图）与已是 WebP 的原样保留
+	ImageMaxBytes = 5 << 20 // 5 MiB（对原始上传文件生效）
 	ImageMaxDim   = 4096
 
+	// JPEG 转 WebP 的有损质量；PNG 走无损，不受此参数影响
+	ImageWebPQuality = 80
+
 	// 站点背景图（前台/后台墙纸）
 	BackgroundMaxBytes = 8 << 20 // 8 MiB
 	BackgroundMaxDim   = 8192
@@ -190,7 +194,50 @@ func decodeImageSizeReader(r io.Reader, mime string) (w, h int, err error) {
 	return cfg.Width, cfg.Height, nil
 }
 
-// SaveImage 流式保存帖子插图：校验格式/大小/尺寸 → 落盘 → 写 attachments（kind=image）
+// transcodeImageToWebP 把校验通过的 JPEG/PNG 临时文件解码后转码为 WebP：
+// JPEG 走有损 q80，PNG 走无损（保留透明通道与文字锐度）。
+// 成功返回新文件名/MIME/大小并清理原临时文件；失败返回 error（调用方保留原格式落盘）。
+func (s *UploadService) transcodeImageToWebP(tmp, name, ext string) (string, string, int64, error) {
+	webpName := strings.TrimSuffix(name, ext) + ".webp"
+	webpPath := filepath.Join(s.dir, "images", webpName)
+	webpTmp := webpPath + ".partial"
+	if s.ops != nil {
+		release, e := s.ops.BeginTemporary(webpTmp)
+		if e != nil {
+			return "", "", 0, e
+		}
+		defer release()
+	}
+
+	f, err := os.Open(tmp)
+	if err != nil {
+		return "", "", 0, err
+	}
+	img, _, err := image.Decode(f) // 仅 JPEG/PNG 会走到这里，解码器已在文件头注册
+	_ = f.Close()
+	if err != nil {
+		return "", "", 0, err
+	}
+	var buf bytes.Buffer
+	if err := webpenc.Encode(&buf, img, webpenc.Options{
+		Quality:  ImageWebPQuality,
+		Lossless: ext == ".png",
+	}); err != nil {
+		return "", "", 0, err
+	}
+	if err := os.WriteFile(webpTmp, buf.Bytes(), 0o644); err != nil {
+		_ = os.Remove(webpTmp)
+		return "", "", 0, err
+	}
+	if err := os.Rename(webpTmp, webpPath); err != nil {
+		_ = os.Remove(webpTmp)
+		return "", "", 0, err
+	}
+	_ = os.Remove(tmp)
+	return webpName, "image/webp", int64(buf.Len()), nil
+}
+
+// SaveImage 流式保存帖子插图：校验格式/大小/尺寸 → JPEG/PNG 转 WebP → 落盘 → 写 attachments（kind=image）
 func (s *UploadService) SaveImage(userID uint, src io.Reader) (*model.Attachment, error) {
 	if src == nil {
 		return nil, errors.New("文件为空")
@@ -284,33 +331,48 @@ func (s *UploadService) SaveImage(userID uint, src io.Reader) (*model.Attachment
 		_ = os.Remove(tmp)
 		return nil, errors.New("图片边长不能超过 4096px")
 	}
-	if err := os.Rename(tmp, fullPath); err != nil {
-		_ = os.Remove(tmp)
-		return nil, err
+
+	// JPEG/PNG 统一转存 WebP；GIF（动图）与已是 WebP 的原样保留
+	storeName, storeMime, storeSize := filename, format.mime, total
+	storePath := fullPath
+	if format.mime == "image/jpeg" || format.mime == "image/png" {
+		name, mime, size, err := s.transcodeImageToWebP(tmp, filename, format.ext)
+		if err != nil {
+			log.Printf("[upload] 插图转 WebP 失败，保留原格式 %s: %v", filename, err)
+		} else {
+			storeName, storeMime, storeSize = name, mime, size
+			storePath = filepath.Join(s.dir, "images", name)
+		}
+	}
+	if storePath == fullPath {
+		if err := os.Rename(tmp, fullPath); err != nil {
+			_ = os.Remove(tmp)
+			return nil, err
+		}
 	}
 
 	att := &model.Attachment{
 		UserID: userID,
 		Kind:   model.AttachmentKindImage,
-		URL:    "/uploads/images/" + filename,
-		MIME:   format.mime,
-		Size:   int(total),
+		URL:    "/uploads/images/" + storeName,
+		MIME:   storeMime,
+		Size:   int(storeSize),
 		Width:  w,
 		Height: h,
 	}
 	if s.ops != nil {
-		id, e := s.ops.StoreFile(fullPath, att.MIME, true)
+		id, e := s.ops.StoreFile(storePath, att.MIME, true)
 		if e != nil {
-			_ = os.Remove(fullPath)
+			_ = os.Remove(storePath)
 			return nil, e
 		}
 		if id != "" {
 			att.URL = "/api/media/" + id
-			_ = os.Remove(fullPath)
+			_ = os.Remove(storePath)
 		}
 	}
 	if err := s.db.Create(att).Error; err != nil {
-		_ = os.Remove(fullPath)
+		_ = os.Remove(storePath)
 		return nil, err
 	}
 	return att, nil