Add image base64 helpers

[treeform]

Summary

• add pixie/imagebase64 with Image.encodeBase64() and decodeBase64()
• decode plain base64 image data or data:*;base64,... URLs
• add dedicated base64 tests and expose helpers through bindings

Closes #550.

Tests

• nim check src/pixie/imagebase64.nim
• nim check src/pixie.nim
• nim r tests/test_base64.nim
• nim r tests/tests.nim