fix: sign-in page now respects global theme

[S-Vignesh07]

Summary

Fixed theme inconsistency where the sign-in page forced dark mode regardless of the user's active theme preference. Now the sign-in page correctly respects the global theme state and matches the landing page appearance.

Closes #1224

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Created custom sign-in page at /auth/signin with theme awareness
• Removed hardcoded dark mode styles (background: "#808808", etc.)
• Integrated sign-in page with existing ThemeContext using useTheme() hook
• Fixed ThemeContext exports (added export keyword to ThemeProvider and useTheme)
• Simplified providers.tsx to only essential providers (SessionProvider + ThemeProvider)
• Sign-in page now dynamically responds to light/dark mode toggles

How to Test

Steps for the reviewer to verify this works:

1. Start the development server (npm run dev)
2. Navigate to landing page and ensure theme is set to Light Mode
3. Click "Sign In" button or go to /auth/signin
4. Expected: Sign-in page displays light theme (white background, dark text)
5. Toggle theme to Dark Mode from the theme switcher
6. Expected: Sign-in page automatically switches to dark theme (dark background, light text)
7. Refresh the page - theme preference persists
8. Navigate directly to /auth/signin - theme correctly applies

Screenshots (if UI change)

Light Mode - Landing Page + Sign-in Page (Both Match)
[Insert screenshot of landing page in light mode + sign-in page in light mode]

Dark Mode - Landing Page + Sign-in Page (Both Match)
[Insert screenshot of landing page in dark mode + sign-in page in dark mode]

Before Fix - Jarring Transition
[Optional: Show landing page light mode → sign-in page dark mode]

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable (N/A - UI/theme change only)

[vercel]

@S-Vignesh07 is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[github-actions]

Thanks for your first PR on DevTrack! 🎉

A maintainer will review it within 48 hours. While you wait:

• Make sure CI is passing (type-check + lint)
• Double-check the PR description is filled out and the issue is linked
• Feel free to ask questions in Discussions if you need help

If you find DevTrack useful, a ⭐ star on the repo is always appreciated — it helps the project grow and attract more contributors!