fix(build): override env var with manually-defined localBaseUrl

[kahboom]

This PR fixes an issue with the build and Rekor endpoint. .env.local file was not being picked up for pre-defining NEXT_PUBLIC_REKOR_DEFAULT_DOMAIN at runtime, and user-defined endpoints in the UI (via Settings modal) were further not overriding that default value.

Changes:

• Revert react-hook-form changes for sanitization of the endpoint URL, and instead use PatternFly and validation natively as we were before, because it affects how baseUrl is used in React Context
• Use process.env instead of getConfig from next.js, which also seems unreliable
• Incorporate some of Masha (UXD)'s feedback request to only validate on submit

[codecov]

Codecov Report

Attention: Patch coverage is 72.28916% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 64.30%. Comparing base (4cd9ab6) to head (915ed91).

❗ Current head 915ed91 differs from pull request most recent head 8da94d6. Consider uploading reports for the commit 8da94d6 to get more accurate results

Files	Patch %	Lines
src/modules/components/Settings.tsx	72.83%	22 Missing ⚠️
src/modules/api/context.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   65.91%   64.30%   -1.61%     
==========================================
  Files          14       14              
  Lines        1590     1569      -21     
  Branches       59       43      -16     
==========================================
- Hits         1048     1009      -39     
- Misses        542      560      +18     

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