Auth: Mobile — SecureStore token management + API interceptor

[EricGrill]

Auth System — Mobile Step 2

Secure token storage and automatic token refresh for the mobile app.

Token storage (expo-secure-store):

• Refresh token → SecureStore (iOS Keychain / Android Keystore)
• User profile → SecureStore (cached for offline access)
• Access token → React state only (memory)

API interceptor (mobile/utils/api.ts):

• Attach Authorization: Bearer <token> to all requests
• On 401 → attempt refresh from SecureStore → retry or navigate to LoginScreen
• On network timeout → keep user authenticated with cached profile, retry on connectivity

Offline handling:

• Use @react-native-community/netinfo to detect connectivity
• If refresh fails due to network (not 401), stay authenticated
• Queue refresh retry for when connectivity returns

Dependencies: expo-secure-store, @react-native-community/netinfo

Depends on: Mobile AuthStack
Design doc: docs/AUTH.md

[EricGrill]

Implemented in PR #152.