fix: prevent duplicate visible guild names

[darbyjack]

Summary

Prevent guild creation and renaming from accepting a name that differs from an existing guild only by color formatting or letter case.

Root cause

Guild names are stored after translating alternate color codes, while the duplicate-name check compared the stored formatted string directly against the raw command input. This allowed visually identical names such as &aKnights and &bKnights to coexist.

Changes

• add a shared visible-name uniqueness check to GuildInputValidator
• translate alternate color codes and remove formatting from both existing and proposed names before comparison
• keep the comparison case-insensitive
• use the normalized check during guild creation and guild rename
• leave name storage and display formatting unchanged

Local validation

Compiled the exact validator locally with lightweight dependency stubs and ran standalone behavior checks covering:

• different & colors on the same visible name
• existing §-formatted names
• case-only differences
• formatting-only differences
• genuinely distinct names and suffixes