fix(account): repair OAuth social login end-to-end (#11)

[fupelaqu]

• OAuth2Service.userInfo: drop null JSON values before toString to stop NPE on providers returning nullable profile fields (e.g. GitHub /user)
• GitHubApiService: fall back to GET /user/emails for the primary verified email when /user returns a private (null) email, so login resolves
• add post-auth redirect: callback now 302s to a configurable frontend URL (auth.oauth.callback.redirect-url / AUTH_OAUTH_CALLBACK_REDIRECT_URL) with status=success|error[&reason], session cookie set on the same response; mirrored in both tapir (OAuthServiceEndpoints) and akka-http (OAuthService)
• bump version to 0.8.6

[codecov]

Codecov Report

❌ Patch coverage is 35.89744% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.49%. Comparing base (b7d9e6e) to head (8dc975f).

Files with missing lines	Patch %	Lines
...app/softnetwork/account/spi/GitHubApiService.scala	0.00%	15 Missing ⚠️
...etwork/account/service/OAuthServiceEndpoints.scala	23.07%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   74.72%   74.49%   -0.23%     
==========================================
  Files          55       55              
  Lines        2568     2596      +28     
  Branches      407      403       -4     
==========================================
+ Hits         1919     1934      +15     
- Misses        649      662      +13     

Flag	Coverage Δ
unittests	74.49% <35.89%> (-0.23%)	⬇️

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