Feature - Add Subscription Management Page

[iz-120]

Closes: #9

This pull request introduces the subscription management page to the user area, accessible via the sidebar navigation. The page allows users to view and manage their current subscription plan, payment method, available plans, and usage statistics.

Frontend Improvements

subscription-management.tsx

• Created the subscription management page with three main tabs:
  • Plano atual: Shows the user's current plan details and payment method, with both cards displayed side by side and responsive for smaller screens.
  • Utilização: Displays usage statistics in styled cards, including chat credits, knowledge bases, and storage, each with progress bars and icons, following the application's visual style.
  • Planos disponíveis: Lists all available plans in cards, highlighting the current plan with a disabled button labeled "Plano atual" and active buttons for other plans.
• Sidebar navigation updated to include the "Assinatura" (subscription) section, keeping the user experience consistent across the user area.
• All components and layout follow the design system and styling conventions already established in the project.

Styling and Utilities

• Cards, buttons, and progress bars use the same Tailwind and component classes as the rest of the user area for a seamless look.
• Responsive design ensures usability on both desktop and mobile.
• The structure is ready for future integration with backend data and further feature expansion.

---

This PR lays the foundation for subscription management in the user area, maintaining visual and UX consistency with the rest of the application. It should be updated once we all decide how the pricing will be determined and calculated.