[mobile] Add Expo app config (app.json, eas.json, icons/splash) to unblock APK builds

[Dipraise1]

Context

mobile/ is an Expo + React Native app with screens, components, and theme in place, but it cannot produce an APK: there is no app.json (or app.config.ts), no eas.json, and no assets/ directory (icon, adaptive icon, splash).

Tasks

• Add app.json with name, slug, android package id, version, and asset references
• Add assets/ — app icon, adaptive icon, splash screen
• Add eas.json with a preview (APK) and production (AAB) build profile
• Document the build command in mobile/README (eas build -p android --profile preview)

Acceptance criteria

eas build -p android --profile preview completes and produces an installable APK.