Fix blank GitHub sign-in page UI and improve auth layout

[tharunika-19]

Related Issue

Closes #606

Changes Made

• Added a custom NextAuth sign-in page
• Added DevTrack branding and tagline
• Improved GitHub sign-in button visibility and styling
• Replaced the default blank auth screen with a responsive centered auth card
• Improved overall UI/UX of the authentication flow

Before

• Blank dark screen with only a floating GitHub button
• Poor icon/button contrast
• No branding or visual hierarchy

After

• Fully styled and branded authentication page
• Better spacing, contrast, and responsiveness
• Improved user experience and accessibility

Screenshots

Before :

After:

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

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[github-actions]

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[Priyanshu-byte-coder]

Two issues:

1. Missing EOF newline
2. Hardcoded colors — the sign-in page uses hardcoded values:
   • bg-[#0b1120] → bg-[var(--background)]
   • border-white/10, bg-white/5 → border-[var(--border)], bg-[var(--card-muted)]
   • text-white → text-[var(--foreground)]
   • text-gray-400 → text-[var(--muted-foreground)]
   • bg-white text-black (GitHub button) — the GitHub sign-in button can keep a neutral style, but use bg-[var(--background)] text-[var(--foreground)] to respect theme

[tharunika-19]

Thanks for the review! 🚀

I’ve updated the sign-in page to use theme variables instead of hardcoded colors, added the missing EOF newline, and improved the button hover interaction as well.