Auth: Mobile — Login + Register screens (AuthStack)

[EricGrill]

Auth System — Mobile Step 1

Replace the hardcoded role-picker with real login/registration screens.

New screens:

• mobile/screens/auth/LoginScreen.tsx — email/phone + password
• mobile/screens/auth/RegisterScreen.tsx — invite-based registration
• mobile/screens/auth/ForgotPasswordScreen.tsx — password reset

Navigation change:

Root Navigator
├── AuthStack (unauthenticated)
│   ├── LoginScreen
│   ├── RegisterScreen
│   └── ForgotPasswordScreen
└── AppStack (authenticated)
    ├── WorkerStack
    ├── ForemanStack
    ├── FarmStack
    └── SupplyStack

Remove: current role-picker in App.tsx, hardcoded user IDs

Auth state on app launch:

1. Show splash screen
2. Check SecureStore for refresh token
3. If found → call /api/auth/refresh → success → AppStack
4. If missing or failed → AuthStack

Active stack determined by user's highest-privilege role at selected farm (from login response roles array).

i18n: All new screens use existing i18n setup (add keys to en.json/es.json)

Depends on: Backend auth endpoints
Design doc: docs/AUTH.md