#366 feat(desktop): replace keychain storage with encrypted file storage

Summary

• Replace Electron's safeStorage API with custom AES-256-GCM encryption
• Eliminates macOS Keychain permission prompts that were confusing users
• Uses machine-derived keys (hardware UUID on macOS, machine-id on Linux, MachineGuid on Windows)

Test plan

• Verify login flow works without Keychain prompt on macOS
• Verify tokens persist across app restarts
• Verify tokens are encrypted in ~/.superset/auth-session.enc

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Stronger, machine-bound encryption for locally stored authentication tokens for improved security and portability across platforms.
• Behavior Changes
  • Token storage now always encrypts session data on disk rather than relying on platform keychains; loading relies on the encrypted file being readable and decryptable.

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