Refactor: Extract shared AnimatedStarsBackground component

[coderabbitai]

Issue Description

There is duplicated star animation code between the SignIn page (apps/web/src/app/signin/page.tsx) and SignUp page that should be extracted into a reusable component.

Current Problem

The star animation JSX code is duplicated across multiple authentication pages, leading to code duplication and maintenance overhead.

Proposed Solution

Create a reusable AnimatedStarsBackground component that:

• Encapsulates star positions, sizes, and animation delays in a data array
• Maps over the array to render StarIcon elements
• Can be imported and used across different pages

Implementation Details

The component should be created at components/ui/AnimatedStarsBackground.tsx and replace the duplicated star JSX in both signin and signup pages.

References

• PR: Refactor UI components and enhance Supabase integration #65
• Comment: Refactor UI components and enhance Supabase integration #65 (comment)
• Requested by: @daemon-node-byte

Acceptance Criteria

• Create AnimatedStarsBackground component
• Replace duplicated code in signin page
• Replace duplicated code in signup page
• Ensure visual behavior remains identical
• Add appropriate TypeScript types