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.

Tests:

• git diff --check
• Temporary C++ harness against simdutf v9.0.0 covering selected DataUrl/atob edge cases: extra padding, interior '=', garbage bytes, Unicode bytes, and base64url characters
• Not run: bazel test targets locally because bazel is not installed on this PATH

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