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/service/server.go

@@ -936,7 +936,7 @@ func (s *ServerService) fetchXrayDigestSHA256(client *http.Client, dgstURL strin
 // parseXrayDigestSHA256 extracts the lowercase SHA2-256 hex from an XTLS .dgst
 // file, whose lines are "ALGO= <hex>" (the relevant one being "SHA2-256= ...").
 func parseXrayDigestSHA256(dgst []byte) (string, error) {
-	for _, line := range strings.Split(string(dgst), "\n") {
+	for line := range strings.SplitSeq(string(dgst), "\n") {
 		rest, ok := strings.CutPrefix(strings.TrimSpace(line), "SHA2-256=")
 		if !ok {
 			continue