[otel] Log UI language from the backend

[imnasnainaec]

Alternative to #4062

Triggered by login
Uses Request.Headers.AcceptLanguage to infer UI language when not explicitly set by the user...
Thus, can return languages that we don't actually have available, which can be mapped to English in analytics processing
Also, doesn't track when the user changes UI language until the next time they log in

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch otel-ui-lang

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[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.47%. Comparing base (82bee05) to head (c1be9b1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
Backend/Controllers/UserController.cs	14.28%	11 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4063       +/-   ##
===========================================
+ Coverage   74.59%   85.47%   +10.87%     
===========================================
  Files         294       54      -240     
  Lines       10908     4778     -6130     
  Branches     1367      591      -776     
===========================================
- Hits         8137     4084     -4053     
+ Misses       2375      550     -1825     
+ Partials      396      144      -252     

Flag	Coverage Δ
backend	85.47% <14.28%> (-0.20%)	⬇️
frontend	?

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[imnasnainaec]

#4062 preferred