fix(http): guard top-level Deno global access for browser compatibility

[Asguho]

Problem:
When importing UserAgent or other utilities from @std/http in a browser environment (e.g., via Vite), the application crashes. This occurs because file_server.ts performs top-level execution of code accessing the Deno global namespace (specifically Deno.permissions and Deno.env) which does not exist in the browser.
Solution:
This PR adds a typeof Deno !== "undefined" check before attempting to access permission queries or environment variables. This ensures the module can be loaded in non-Deno environments without throwing a ReferenceError.

If there is anything else I can do to help this merge, please let me know.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.19%. Comparing base (1cd85bf) to head (c66e55e).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
http/file_server.ts	42.85%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6987      +/-   ##
==========================================
- Coverage   94.20%   94.19%   -0.01%     
==========================================
  Files         613      613              
  Lines       47710    47715       +5     
  Branches     8299     8299              
==========================================
+ Hits        44945    44946       +1     
- Misses       2697     2701       +4     
  Partials       68       68              

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[kt3k]

LGTM. Thanks!