FIX: preserve original Codeforces username casing after login

[PratikBhopi]

Fixes : #9

Why is this change needed?

• Codeforces handles can contain mixed-case characters, and converting them to lowercase:
• Creates inconsistency with the official handle
• Can confuse users when viewing profiles
• Breaks visual identity across platforms

How it was fixed?

• Preserved the original username casing returned by Codeforces
• Continued using a lowercase version internally only where needed (e.g. comparisons or lookups).

Before :

Fixed:

[vercel]

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