Migrate frontend from Next.js to Vite + React Router

- Replace Next.js App Router with react-router-dom v7 SPA routing
- Remove all 'use client' directives, next/navigation, next/link imports
- Replace useRouter/usePathname with useNavigate/useLocation
- Replace useSearchParams() call with [searchParams] destructuring
- Add Amplify <Authenticator> as root auth gate, remove custom login/signup pages
- Add AuthProvider inside Authenticator render in App.tsx
- Consolidate Amplify config (including Identity Pool) in lib/aws/config.ts
- Build AgentCore invocation URLs from ARNs at runtime
- Replace NEXT_PUBLIC_* env vars with VITE_* throughout
- Replace uploadData key param with path to avoid public/ prefix
- Add key={id} wrapper on AssistantChatPage for unmount/remount on route change
- Replace process.env.NEXT_PUBLIC_API_BASE_URL with API_URL from config
- Account page uses React Router nested routes for /account/models
- WorkflowPage migrated from next/dynamic to direct import
- TypeScript compiles clean, Vite build succeeds