Fix UTF-8 Base64 decoding corruption in GitHub files

[Akshay10258]

Fix Description

Fixes issue - #5255
Resolves UTF-8 character corruption in GitHub file loading by replacing naive Base64 decoding with UTF-8 aware decoding.

Testing

• Verified fix using browser console testing.
• Characters like 👨‍💻 now load correctly instead of showing corruption - ������.
• Includes fallback for compatibility

Code Review Checklist

• ✅ Implements intended functionality (UTF-8 fix)
• ✅ Handles error scenarios (try-catch with fallback)
• ✅ Follows existing code patterns
• ✅ No performance impact (minimal overhead)
• ✅ Maintains backward compatibility

[davidjgraph]

Thanks for the suggestion. Unfortunately, this isn't the right place to fix the root of the problem, we've done that for the next release.