Feature/mobileapp 583 privy social login

[Ova-Klik]

Closes #583

Implements a Privy-branded social login experience for the mobile application, providing users with a clean onboarding entry point and support for multiple authentication methods.

Changes Made

• Added a modern social login screen in mobileapp/app/index.tsx
• Added onboarding start screen in mobileapp/app/onboarding-start.tsx
• Integrated Privy authentication widgets
• Added social sign-in options:
  • Google
  • Apple
  • Email
• Configured interaction handlers to launch Privy browser overlays
• Applied responsive, branded UI styling for a polished onboarding experience

Testing

Verified the following scenarios:

• ✅ Login screen renders correctly
• ✅ Google button launches the Privy authentication flow
• ✅ Apple button launches the Privy authentication flow
• ✅ Email button launches the Privy authentication flow
• ✅ Layout is responsive across supported mobile screen sizes
• ✅ Buttons provide appropriate loading/interaction feedback

Acceptance Criteria

• Render clean social connection buttons
• Launch Privy browser overlays on interaction
• Apply sleek styling consistent with project branding

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[ONEONUORA]

Great job @Ova-Klik