test: add Base64 and URL encoder coverage

[swetha-loco]

Summary

• Extracted the Base64/URL encoder logic into a small utility so it can be tested directly.
• Added frontend coverage for Base64 and URL encoding/decoding.
• Covered round trips, invalid input handling, empty input, unicode, spaces, +, and % edge cases.
• Added test:encoder and included it in CI.

Tests

• npm run test:encoder
• npm run test:urls
• npm run test:i18n
• npx tsc --noEmit
• npm run lint
• pytest tests/ -v --basetemp=.pytest-tmp

Closes #143