refactor: add cross-environment decodeBase64 method

[Phillip9587]

Based on #93

This PR decouples the package from Node.js APIs and adds support for browser and other runtimes. Base64 decoding now uses runtime feature detection:

1. Uint8Array.fromBase64() when available (modern browsers/runtimes)
2. Buffer.from(..., 'base64') in Node.js and compatible runtimes
3. custom implementation using atob() and TextDecoder as a fallback

[codecov]

Codecov Report

❌ Patch coverage is 41.17647% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.74%. Comparing base (c0d7950) to head (304497e).

Files with missing lines	Patch %	Lines
src/index.ts	41.17%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##            master      #95       +/-   ##
============================================
- Coverage   100.00%   76.74%   -23.26%     
============================================
  Files            1        1               
  Lines           30       43       +13     
  Branches        11       13        +2     
============================================
+ Hits            30       33        +3     
- Misses           0       10       +10     

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[blakeembrey]

Will need rebasing too, but would you be able to add a benchmarks folder with the different ways to decode for posterity? That way we can quickly re-run benchmarks anytime and confirm we've got the right methods selected.

[Phillip9587]

@blakeembrey I added the benchmarks and switched the Uin8Array.fromBase64 and Node.js Buffer decode methods in the cross-environment decodeBase64() function because it looks like the Uint8Array.fromBase64() + TextDecoder method is still significantly slower than the Buffer version on Node.js v25.9.0

 ✓ benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > short username/password 3494ms
     name                                            hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · Uint8Array.fromBase64 + TextDecoder   1,989,391.31  0.0003  3.1667  0.0005  0.0005  0.0019  0.0025  0.0044  ±1.39%   994696
   · Buffer.from(base64).toString(utf-8)   6,569,899.93  0.0001  0.4416  0.0002  0.0002  0.0003  0.0005  0.0012  ±0.35%  3284950
   · atob + Uint8Array.from + TextDecoder  1,282,955.78  0.0007  0.3789  0.0008  0.0008  0.0014  0.0015  0.0044  ±0.29%   641478

 ✓ benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > common API style credential 3231ms
     name                                            hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · Uint8Array.fromBase64 + TextDecoder   1,856,823.64  0.0003  1.4238  0.0005  0.0006  0.0018  0.0022  0.0049  ±1.14%   928412
   · Buffer.from(base64).toString(utf-8)   6,761,025.03  0.0001  0.2447  0.0001  0.0001  0.0004  0.0006  0.0012  ±0.27%  3380513
   · atob + Uint8Array.from + TextDecoder    414,072.23  0.0022  0.3207  0.0024  0.0024  0.0033  0.0057  0.0063  ±0.29%   207037

 ✓ benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > long token-like password 3170ms
     name                                            hz     min      max    mean     p75     p99    p995    p999     rme  samples
   · Uint8Array.fromBase64 + TextDecoder   1,774,143.02  0.0003   2.4190  0.0006  0.0006  0.0018  0.0021  0.0040  ±1.45%   887073
   · Buffer.from(base64).toString(utf-8)   5,983,805.38  0.0001  20.8329  0.0002  0.0002  0.0006  0.0008  0.0016  ±8.17%  2991903
   · atob + Uint8Array.from + TextDecoder    256,825.88  0.0033   0.2730  0.0039  0.0039  0.0054  0.0070  0.0098  ±0.26%   128413

 BENCH  Summary

  Buffer.from(base64).toString(utf-8) - benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > short username/password
    3.30x faster than Uint8Array.fromBase64 + TextDecoder
    5.12x faster than atob + Uint8Array.from + TextDecoder

  Buffer.from(base64).toString(utf-8) - benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > common API style credential
    3.64x faster than Uint8Array.fromBase64 + TextDecoder
    16.33x faster than atob + Uint8Array.from + TextDecoder

  Buffer.from(base64).toString(utf-8) - benchmarks/base64.bench.ts > decode base64 for basic-auth payloads > long token-like password
    3.37x faster than Uint8Array.fromBase64 + TextDecoder
    23.30x faster than atob + Uint8Array.from + TextDecoder

[Phillip9587]

@blakeembrey Any suggestions on how to fix the test coverage?

[blakeembrey]

We seem to be doing this in a few packages now, wdyt about a new package for base64? Or can we benchmark some existing ones too?

[Phillip9587]

Which other packages need base64?

[blakeembrey]

I think I've misremembered, I thought we were doing it another package but I don't see anything.

As for improving coverage, how about a base64.ts file, export each implementation, and run the tests that they all align. The default export for the package can just be the "selector" based on environment. That won't let you test which one is selected, but at least the implementations should be covered.

[blakeembrey]

If you do add a separate file, we could also have it be import specific for browsers vs node vs other, instead of relying on a runtime check it'd rely on the bundler using the correct file.