refactor: extract SHA3 hashing utilities to utils/hash.go

- Move all SHA3-512 hashing functions to utils package
- Update import statements and function calls
- Maintain zero functional changes
- First step in systematic main.go refactoring

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

utils/hash.go

@@ -0,0 +1,25 @@
+package utils
+
+import (
+	"encoding/hex"
+	"golang.org/x/crypto/sha3"
+)
+
+// SHA3-512 hashing utilities for Phase 2 authentication
+func HashSHA3512(input string) string {
+	hasher := sha3.New512()
+	hasher.Write([]byte(input))
+	return hex.EncodeToString(hasher.Sum(nil))
+}
+
+func HashUserNickname(nickname string) string {
+	return HashSHA3512(nickname)
+}
+
+func HashGroupName(groupname string) string {
+	return HashSHA3512(groupname)
+}
+
+func HashToken(token string) string {
+	return HashSHA3512(token)
+}