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/web.go

@@ -429,7 +429,7 @@ func (s *Server) memoryAlarmWanted() bool {
 		if threshold <= 0 {
 			return false
 		}
-		for _, e := range strings.Split(events, ",") {
+		for e := range strings.SplitSeq(events, ",") {
 			if strings.TrimSpace(e) == string(eventbus.EventMemoryHigh) {
 				return true
 			}