feat: add theme toggle to public profile page

[inimai09]

Summary

Adds the existing ThemeToggle component to the public profile page header
so visitors can switch between dark and light mode.

Closes #1032

Type of Change

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

Changes Made

• Added ThemeToggle import to src/app/u/[username]/page.tsx
• Wrapped existing StatsCard in a flex div alongside ThemeToggle

How to Test

1. Visit public profile page /u/[username]
2. Toggle should be visible in the top-right corner next to the download card button
3. Click the toggle — page should switch between dark and light mode immediately
4. Refresh the page — preference should persist (saved in localStorage)
5. Test without being logged in — toggle should still work

Screenshots (if UI change)

Toggle appears in top-right of profile header alongside StatsCard

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

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[github-actions]

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