Enhance the appearance and branding of the main application dashboard

Refactors `App.tsx` to improve the UI, incorporating a gradient background, professional header with a shield icon, and a prominent trial banner.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: d5daeb82-020f-4ea2-a54f-9097efac2a63
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/4119c42c-9857-448a-8c94-20be047485ba/aeba0db3-52ed-4cfd-9b6e-5f3d43768478.jpg