Refactor/signin register layout

[mariukle]

This PR refactors the Sign In and Register pages to improve layout,
styling and user experience.

Changes include:

• Updated color theme to match the rest of the application
• Added field labels for better usability
• Resized components to scale properly for fullscreen
• Moved shared CSS classes (confirm-button, field-label, input-field) to Global.css
• Placed password fields side by side on the Register page
• Removed unimplemented "Forgot your password?" link