fix: improve handling of disabled accounts

[Anty0]

Summary

• Disabled accounts are now properly detected during login, sign-up, and third-party auth flows
• Login returns a specific user_account_disabled error instead of generic "bad credentials"
• Sign-up detects disabled accounts gracefully instead of causing a database constraint violation
• Added findActiveOrDisabled() repository method that finds non-deleted users regardless of disabled status
• Frontend maps the new error code to a user-friendly message directing users to contact their org owner or server admin

Closes #3276

Summary by CodeRabbit

• New Features

  • Enhanced authentication flows with improved error handling for disabled accounts. Users attempting to sign up or log in with a disabled account now receive a clear, specific error message indicating the account is disabled.

• Tests

  • Added tests for disabled account authentication scenarios.

[coderabbitai]

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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 jirikuchynka/fix-disabled-accounts

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