Update UI for headscale 0.28 API

[ChowDPa02k]

Summary

This PR updates the UI for Headscale v0.28 and refreshes the README to reflect the current API direction and support
status.

The implementation and validation for this change were completed with Codex using GPT-5.4.

What changed

• switched client node operations to the /api/v1/node API used by Headscale v0.28
• removed legacy /api/v1/machine/... branches from the Angular API service
• updated user operations to use user IDs where required by newer handlers
• updated pre-auth key operations to match current request and response shapes
• replaced legacy route management calls with node approved-route updates
• changed the login default to v0.28
• improved login error handling
• updated the README for current compatibility and limitations

This is not a relocation of the old machine API implementation. The legacy machine endpoint branches were removed, and
the client now standardizes on the newer node API flow.

In v0.28 mode, tag editing is currently treated as read-only in the machine view.

Validation

Validated against Headscale v0.28 for:

• login
• user list / rename / delete
• node list
• node register / rename / expire / delete
• route approval updates
• pre-auth key list / create / expire