fix(auth): clear in-memory loans on sign-out

[EmeditWeb]

Summary

Closes #41

Sign-out cleared the wallet session, user profile, and auth tokens, but the loans store stayed populated in memory. On a shared device the next account could briefly see the previous user's loans until a refresh replaced them. This adds clearLoans() to both full sign-out paths, completing the sign-out teardown noted as the remaining follow-up on #41 (the WalletConnect-disconnect core was already resolved).

This repo is for the React Native mobile app only

This app targets learners only.
Sponsor, vendor, and mentor features belong
in StepFi-Web, not here.

Before submitting, confirm your changes belong here:

• My changes are React Native components
  or Expo code
• I have NOT added web-only APIs
  (window, document, localStorage)
• I have NOT hardcoded hex color values
  (use constants/colors.ts only)
• All icons are from lucide-react-native only
• No API calls made directly in screen files
  (use services/ layer only)
• Every screen has loading, error,
  and empty states

Type of change

• Bug fix
• New screen
• New component
• Service or store change
• Navigation change
• Performance or offline improvement

Testing

• npx expo export --platform web passes
• No TypeScript errors
• Tested on Android emulator or physical device
• No hardcoded hex colors
• No console errors

Context files reviewed

• context/architecture-context.md
• context/code-standards.md

Mandatory before requesting review

This must exit 0:
npx expo export --platform web

Verified: npx expo export --platform web exits 0; tsc --noEmit clean; npm test 26/26 passing; npm run lint 0 errors.