fix: 修复三个空白页面 + 许可证页面重构

- 修复发现页空白：后端 discover.go nil→[] Match，前端 discover.ts 加 ?? []
- 修复下载客户端页空白：后端 download_clients.go nil→[]model.DownloadClient，前端加 ?? []
- 修复通知渠道页空白：后端 notify_channels.go nil→[]struct{}，前端加 ?? []
- 许可证页面重构：移除生成密钥功能，对接 LicenseServer 绑定/状态模式
- 所有变更通过 Go build + npm run build 零错误验证

internal/handler/discover.go

@@ -16,6 +16,9 @@ func trendingHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
+		if items == nil {
+			items = []service.Match{}
+		}
 		c.JSON(http.StatusOK, gin.H{"items": items})
 	}
 }
@@ -27,6 +30,9 @@ func popularHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
+		if items == nil {
+			items = []service.Match{}
+		}
 		c.JSON(http.StatusOK, gin.H{"items": items})
 	}
 }

internal/handler/download_clients.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 
+	"github.com/ShukeBta/MediaStationGo/internal/model"
 	"github.com/ShukeBta/MediaStationGo/internal/service"
 )
 
@@ -17,6 +18,9 @@ func listDownloadClientsHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
+		if rows == nil {
+			rows = []model.DownloadClient{}
+		}
 		c.JSON(http.StatusOK, rows)
 	}
 }

internal/handler/notify_channels.go

@@ -16,6 +16,10 @@ func listNotifyChannelsHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
+		if rows == nil {
+			c.JSON(http.StatusOK, []struct{}{})
+			return
+		}
 		c.JSON(http.StatusOK, rows)
 	}
 }