渠道: 移除主页/favicon 特性, Base URL 可选+完整地址

- 回退渠道 homepage/favicon 字段与代理接口(未采用)
- Base URL 改为可选: 留空按供应商默认(openai/anthropic),
  兼容兼容型渠道必须填; 填完整地址(含 /v1)时归一化去尾
- 网关 upstreamURL 兜底去 /v1, 避免路径重复

Co-Authored-By: Claude <noreply@anthropic.com>

server/internal/api/router.go

@@ -93,7 +93,6 @@ func NewRouter(a *app.App, gw *proxy.Gateway) *gin.Engine {
 			admin.DELETE("/channels/:id", h.AdminDeleteChannel)
 			admin.POST("/channels/:id/test", h.AdminTestChannel)
 			admin.POST("/channels/:id/models/import", h.AdminImportChannelModels)
-			admin.GET("/channels/:id/favicon", h.AdminChannelFavicon)
 			// 模型与定价
 			admin.GET("/models", h.AdminModels)
 			admin.POST("/models", h.AdminCreateModel)