refactor: Apply modern C# and TypeScript syntax improvements

Applies Copilot suggestions for code modernization:

C# Changes:
- Replace Substring(0, n) with range indexer [..n] syntax
- More concise and idiomatic for .NET 8

TypeScript Changes:
- Eliminate intermediate Array allocation in base64 to Uint8Array conversion
- Directly create Uint8Array and populate in loop for better performance

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

Author: liamgold