Use simdutf for remaining base64 decodes

[Brooooooklyn]

Summary:

• Replace global atob() decoding with simdutf::base64_to_binary while preserving the existing InvalidCharacterError behavior.
• Replace DataUrl's KJ base64 decode with a compatibility-preserving base64 alphabet filter followed by simdutf decoding.
• Update the data-url Bazel target to depend directly on @simdutf and drop the unused encoding implementation dependency.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 129 skipped benchmarks¹

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_{Comparing Brooooooklyn:optimize-simd-base64-decode (40c9392) with main (1bdc662)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Brooooooklyn]

I have read the CLA Document and I hereby sign the CLA

[anonrig]

lgtm