Shadcn UI for a consistent, responsive, and accessible UI across the Next.js project

Review the entire Next.js codebase in `app/` and `src/` to identify components not using Shadcn UI. Update all components to use Shadcn UI (e.g., replace custom `<button>` with `<Button>` from `src/components/ui/button.tsx`, `<input>` with `<Input>` from `src/components/ui/input.tsx`). Ensure the following: - Use TypeScript for type safety with explicit prop interfaces. - Maintain responsive design with Tailwind CSS utilities (`sm:`, `md:`, `lg:`) for consistent styling across devices. - Centralize Shadcn UI components in `src/components/ui/` to avoid duplication. - Remove custom or redundant CSS in favor of Shadcn UI and Tailwind CSS. - Ensure accessibility with ARIA attributes and keyboard navigation. - Simplify component logic to avoid complexity (e.g., use simple hooks like `useState` instead of custom hooks where possible). - Update related SSR pages in `app/` to reflect changes. - Handle errors gracefully in the UI using Shadcn UI’s `<Alert>` for error states. - Log all updated files and removed code for review. - Trigger `npm run build` to verify the updated codebase.