Enhance Login and Signup UI for Improved User Experience

**Overview:**
The login and signup screens are crucial components of our Flutter chat app, and their design and user experience are essential for user engagement. In this issue, we aim to enhance the visual appeal and usability of these screens to create a more welcoming and efficient user onboarding process.
**Tasks:**
**Design Refinement:** Improve the visual design of the login and signup screens to make them more visually appealing and cohesive with the app's overall aesthetic.
**User-Friendly Elements:** Implement user-friendly elements such as clear input field labels, informative placeholders, and validation messages to guide users during the login and signup process.
**Responsive Design:** Ensure that the UI is responsive to different screen sizes and orientations, guaranteeing a consistent and pleasing user experience on various devices.
**Accessibility:** Enhance accessibility features, including proper contrast ratios and screen reader support, to make the app more inclusive.
**Consistency:** Ensure a consistent design language is applied to both the login and signup screens to provide a unified user experience.
**Animations:** Add subtle animations or transitions to create a more engaging and polished user interface.
**Additional Information:**
To get started with this issue, fork the repository and create a new branch for your work. Make sure to test your changes on different devices and screen sizes to confirm that the UI improvements are effective and responsive. Once you've made your improvements, submit a pull request for review.