Use efficient APIs and simplify loops

Minor refactors across the codebase to improve readability and use more efficient APIs: replace fmt.Sprintf+base64 encoding with fmt.Appendf when building Shadowsocks userInfo; compute elapsed using max(now-prev.at, window) to simplify logic; use strings.SplitSeq for splitting in two places; simplify test and goroutine loops to range-based iterations and use errgroup's Go helper; and align/clean up struct field formatting and test map literals. Mostly stylistic/efficiency changes with no intended behavior changes.

internal/web/job/node_traffic_sync_job.go

@@ -246,10 +246,7 @@ func (j *NodeTrafficSyncJob) nodeInboundSpeed() []*xray.Traffic {
 		if dDown < 0 {
 			dDown = 0
 		}
-		elapsed := now - prev.at
-		if elapsed < nodeInboundSpeedWindowMs {
-			elapsed = nodeInboundSpeedWindowMs
-		}
+		elapsed := max(now-prev.at, nodeInboundSpeedWindowMs)
 		up := dUp * nodeInboundSpeedWindowMs / elapsed
 		down := dDown * nodeInboundSpeedWindowMs / elapsed
 		if up > 0 || down > 0 {