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 83.60%. Comparing base (ee70132) to head (2b6d3e0).

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%   83.60%   -16.40%     
============================================
  Files            1        1               
  Lines           48       61       +13     
  Branches        21       23        +2     
============================================
+ Hits            48       51        +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?