chat.z.ai/api/v1/auths/signin returns 400 with 'captcha verification failed'
because the endpoint is designed for browser-only flows with reCAPTCHA.
There is no way to programmatically bypass this from a native Android app.

The correct approach is the API key from Z.AI's dashboard, which is the
official way to authenticate programmatically per their docs.

Changes:
- AIService: remove signIn/signOut/isSignedIn (captcha-protected, unusable)
- AISettingsScreen: restore API key field with clear 5-step instructions
  on how to get a key from z.ai (Settings → API Keys → Create)
- AISettingsViewModel: restore saveKey() and testConnection()