Fixed Issues #55 and #56

[yulmwu]

Title: Fixed Issues #55 and #56

For more details, please refer to the issues below.

• Mirror: Internal Server Error on Duplicate Username During Update in nullforu/smctf #55
• Mirror: Password/Flag Length Limitation Issue During Registration or Challenge Create/Update #56

Since this codebase is aligned with nullforu/wargame, the changes are also almost identical to commit nullforu/wargame@125e4c9.

However, since the frontend code exists separately in smctfe, the frontend changes were updated independently.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/service/user_service.go	63.63%	3 Missing and 1 partial ⚠️
internal/repo/user_repo.go	80.00%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[yulmwu]

Since this is close to a mirror of the wargame codebase, I will not request LLM-based code review for this change.