[Security] Cross-platform credential storage + API key management UI

[jackby03]

Summary

API keys are stored using Windows DPAPI (WindowsDpapiCryptographyService), which only works on Windows. The abstraction ICryptographyService exists but there is no cross-platform fallback and no path to add exchange credentials from the UI.

What needs to be done

• Add macOS/Linux fallback using AesGcm + a machine-derived key stored in the user's config folder
• Add a settings dialog panel in the GUI:
  • List of saved exchange credentials (masked key display)
  • Add / Edit / Delete credential
  • Select active credential
• Validate credential by making a test call to GET /api/v3/account before saving
• Never log or expose raw API keys in console output

Security rules

• Keys must never be stored in plain text
• Keys must never appear in logs, crash reports, or event bus messages
• Use SecureString for in-memory handling where feasible

Acceptance criteria

• WindowsDpapiCryptographyService is used on Windows
• AesGcmCryptographyService is used on macOS and Linux
• Settings dialog allows adding/editing/deleting credentials
• Credential is validated via Binance API before saving
• No raw keys in any log output

References

• src/Omnijure.Core/Shared/Infrastructure/Security/
• src/Omnijure.Core/Features/Settings/Model/ExchangeCredential.cs