Simplify station login with setup tokens

Logging in as a station now uses a single, consistent flow. tofupilot login authenticates you as a user in the browser, and stations register with a setup token from the dashboard.

• Removed the --station login flag in favor of setup tokens for every station.
• Updated the station login errors to point you straight to the dashboard's setup-token page.
• Streamlined the login backend so user and station credentials follow separate, dedicated paths.