add sign in page

[iibarbari]

This pull request includes significant changes to both the frontend and backend of the application. The most important changes involve the addition of a new SignIn component, updates to various CSS files for styling consistency, and a minor update to an error message in the backend.

Frontend Changes:

• New SignIn Component:

  • Added SignIn component with form handling and layout integration (frontend/src/components/SignIn/index.tsx).
  • Created FormTitle sub-component for the SignIn form (frontend/src/components/SignIn/FormTitle/index.tsx).
  • Added CSS modules for SignIn and FormTitle components (frontend/src/components/SignIn/SignIn.module.css, frontend/src/components/SignIn/FormTitle/FormTitle.module.css) [1] [2].

• CSS Refactoring:

  • Reorganized and added new CSS files for various components like buttons, forms, alerts, and containers (frontend/src/styles/components/button.css, frontend/src/styles/components/form.css, frontend/src/styles/components/alert.css, frontend/src/styles/components/container.css) [1] [2] [3] [4].
  • Updated existing components to use the new layout component (frontend/src/components/Error/index.tsx, frontend/src/components/Home/index.tsx) [1] [2].

• Helper Functions:

  • Added a new helper function for base64 encoding (frontend/src/helpers/base64.ts).

Backend Changes:

• Error Message Update:
  • Changed the unauthorized error message from "You are not authorized." to "Invalid credentials." in the Responder component (backend/pkg/restapi/responder/main.go).