test: add unit tests for encodeBase64 utility (#86)

[jaenil]

Closes #86

Summary

Added comprehensive unit tests for encodeBase64 in src/core/utils/base64.ts:

• Empty string input
• Standard ASCII strings (e.g. Basic Auth credentials)
• Zero, one, and two-byte padding variants (Man, Ma, M)
• Multi-byte UTF-8 sequences (accents like é, umlauts ä/ö, and 3-byte Japanese hiragana こんにちは)
• Deterministic output and known test vectors
  Test-only change co-located at src/core/utils/base64.test.ts, no production code touched.

Local Verification

• bun test src/core/utils/base64.test.ts -> 9 pass / 0 fail
• bun run lint -> clean
• bun run typecheck -> clean