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/go.mod

@@ -3,14 +3,18 @@ module github.com/freefire/jiang13-bbs
 go 1.27.0
 
 require (
+	github.com/JohannesKaufmann/html-to-markdown v1.6.0
+	github.com/gen2brain/webp v0.6.4
 	github.com/gin-contrib/cors v1.7.2
 	github.com/gin-gonic/gin v1.10.0
+	github.com/glebarez/sqlite v1.11.0
 	github.com/golang-jwt/jwt/v5 v5.2.2
 	github.com/gorilla/websocket v1.5.3
 	github.com/minio/minio-go/v7 v7.0.83
 	github.com/yuin/goldmark v1.7.8
 	golang.org/x/crypto v0.43.0
 	golang.org/x/image v0.46.0
+	golang.org/x/net v0.45.0
 	golang.org/x/text v0.42.0
 	gopkg.in/ini.v1 v1.67.0
 	gorm.io/driver/postgres v1.5.9
@@ -18,7 +22,6 @@ require (
 )
 
 require (
-	github.com/JohannesKaufmann/html-to-markdown v1.6.0 // indirect
 	github.com/PuerkitoBio/goquery v1.9.2 // indirect
 	github.com/andybalholm/cascadia v1.3.2 // indirect
 	github.com/bytedance/sonic v1.11.6 // indirect
@@ -26,10 +29,10 @@ require (
 	github.com/cloudwego/base64x v0.1.4 // indirect
 	github.com/cloudwego/iasm v0.2.0 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/ebitengine/purego v0.10.1 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.3 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
 	github.com/glebarez/go-sqlite v1.21.2 // indirect
-	github.com/glebarez/sqlite v1.11.0 // indirect
 	github.com/go-ini/ini v1.67.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
@@ -57,7 +60,6 @@ require (
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
 	golang.org/x/arch v0.8.0 // indirect
-	golang.org/x/net v0.45.0 // indirect
 	golang.org/x/sync v0.23.0 // indirect
 	golang.org/x/sys v0.48.0 // indirect
 	google.golang.org/protobuf v1.34.1 // indirect