feat(auth,transport): configurable user-agent, updated origins, operationName

[brewgator]

Summary

• Make auth user-agent configurable via ClientOptions.UserAgent (defaults to monarchmoney-go/1.0.0)
• Update Origin/Referer headers to app.monarch.com (matching Monarch's current domain)
• Extract and send operationName in GraphQL requests (some servers require it)
• Simplify LoginWithEmailOTP — the initial login attempt triggers the OTP email, so this method now just re-submits with the email_otp field
• Expose LoginWithEmailOTP on the AuthService interface
• Include raw response body in BAD_REQUEST errors when no structured error is found

Split from #19 per review feedback.

Test plan

• Existing tests pass (go test ./...)
• Auth flow works with default user-agent
• Auth flow works with custom UserAgent in ClientOptions
• GraphQL requests include operationName field

[codecov]

Codecov Report

❌ Patch coverage is 53.57143% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.44%. Comparing base (56cb2fd) to head (627cc1e).

Files with missing lines	Patch %	Lines
pkg/monarch/auth.go	8.33%	11 Missing ⚠️
internal/transport/graphql.go	87.50%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (53.57%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (46.44%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   42.77%   46.44%   +3.67%     
==========================================
  Files          16       16              
  Lines        1833     1858      +25     
==========================================
+ Hits          784      863      +79     
+ Misses        974      904      -70     
- Partials       75       91      +16     

Flag	Coverage Δ
unittests	46.44% <53.57%> (+3.67%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/monarch/client.go	36.94% <ø> (ø)
internal/transport/graphql.go	63.58% <87.50%> (+40.77%)	⬆️
pkg/monarch/auth.go	10.11% <8.33%> (-0.28%)	⬇️

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[eshaffer321]

Thanks for splitting this up @brewgator. I tested this manually and it looks good. Just fix up the CI errors and this will be good to merge