Add theme selector to landing page

[Yash-Chhipa]

Summary

Closes #2255

---

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

---

What Changed

Added a theme selector to the landing page, allowing users to preview and switch themes before signing in.

• Added ThemeToggle component to the landing page
• Reused existing theme persistence logic
• Verified theme persistence across page refreshes
• Verified theme persistence across route navigation

---

Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

---

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

---

Additional Context

[vercel]

@Yash-Chhipa 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

[Yash-Chhipa]

Hey @Priyanshu-byte-coder, The failing E2E tests appear unrelated to this change. This PR only modifies the landing page selector UI. Failures are occurring in settings (Toggle Public Profile) and streak widget tests.
Kindly review the PR.

[Yash-Chhipa]

Hey @Priyanshu-byte-coder
The landing page visual regression failure is expected because this PR intentionally updates the selector layout on the landing page.

The remaining failing checks appear unrelated to the files modified in this PR:

• settings page saves and reflects changes
• streak widget shows the mocked longest streak value
• public profile screenshot with deterministic mock data

Could a maintainer please review and advise whether these are existing test failures?

[Priyanshu-byte-coder]

The theme toggle addition looks good, but the inner <div> wrapper with maxWidth: 380 and inline styles constrains BentoGrid without clear justification and could break responsive layouts. Please remove the unnecessary wrapper div, fix the indentation, and resubmit.