feat: add custom User-Agent support for subscription and template requests

Add --user-agent / -u CLI flag and user-agent config field to allow
customizing the User-Agent header when fetching subscriptions and
remote templates. This is useful for providers that require a specific
UA to access their subscription links.

Closes #23

api/handler/convert.go

@@ -57,6 +57,7 @@ func Convert(c *gin.Context) {
 		data.SortKey,
 		data.SortType,
 		groupRules,
+		data.UserAgent,
 	)
 	if err != nil {
 		c.JSON(400, gin.H{