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 90.16393% with 6 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@21a5f04). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/index.ts	90.16%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #95   +/-   ##
=========================================
  Coverage          ?   90.16%           
=========================================
  Files             ?        1           
  Lines             ?       61           
  Branches          ?       27           
=========================================
  Hits              ?       55           
  Misses            ?        6           
  Partials          ?        0           

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