feat: Store server token in Keychain

[iret77]

Summary

This implements Issue #42 to enhance security by storing the sensitive server token in the macOS Keychain instead of the insecure UserDefaults.

Changes:

1. Add Dependency: Adds the KeychainAccess library via Swift Package Manager to handle all Keychain operations.
2. Refactor Storage Logic: Replaces all UserDefaults get/set operations for the server token with calls to KeychainAccess. The token is now stored securely, scoped to the app's service identifier.
3. Implement Migration: A transparent migration path is included. On launch, the app checks for a token in the old UserDefaults location. If found, it is moved to the Keychain, and the entry in UserDefaults is deleted. This ensures existing users do not need to re-pair.
4. Add Entitlement: The ClawsyMac.entitlements file has been updated with the "Keychain Sharing" capability to ensure the app has the necessary permissions.

This resolves a high-priority security issue.