feat: add Navbar, Footer, and MainLayout with responsive routing

[Uyoxy]

Implements the persistent app layout with a sticky Navbar and Footer across all public routes.

Changes

• `Navbar.jsx` — sticky, responsive navbar with logo, nav links, auth buttons (Sign In / Get Started), and avatar dropdown menu when logged in. Collapses to hamburger on mobile.
• `Footer.jsx` — four-column footer with Platform, Legal, and Connect (Twitter, Discord, GitHub) sections.
• `MainLayout.jsx` — wraps all pages with Navbar + Footer using ``. Hosts the `` notification system.
• `AppRouter.jsx` — all routes (public, auth, dashboard, admin, 404) moved under a single `<Route element={}>`.

Acceptance Criteria

• Navbar shows Sign In / Get Started when logged out; avatar dropdown when logged in
• Footer present on all public pages
• Layout is fully responsive with hamburger menu on mobile
• Navbar is sticky (stays on scroll)