fix: add request timeout to all fetch calls

[nyxsky404]

📄 Description

All fetch() calls in contributors.js, login.js, and register.js had no timeout, so a slow or unresponsive server would leave users with a permanently spinning UI.

• Added js/fetch-timeout.js — a shared fetchWithTimeout(url, options, ms=10000) helper that wraps fetch() with an AbortController and a configurable timeout (default 10 s).
• Replaced every bare fetch() call in the three affected files with fetchWithTimeout().
• Loaded js/fetch-timeout.js before the consumer scripts in contributors.html, login.html, and register.html.

🔗 Related Issues

Fixes #2238

🧩 Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

✅ Checklist

• Code follows project styling guidelines
• Changes are fully responsive and accessible
• No extraneous logs or debug code left
• Documentation updated accordingly
• Built successfully locally
• Console has zero errors or warnings

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