Implement Dark/Light Mode Theme Persistence

[Sendi0011]

Problem:

Theme preference is not remembered across sessions, and theme switching lacks smooth transitions. Users have to manually switch themes every time they visit.

Files to Modify:

frontend/components/theme-provider.tsx
frontend/components/landing/header.tsx
frontend/app/layout.tsx
frontend/lib/utils.ts

Files to Create:

frontend/components/ui/theme-toggle.tsx
frontend/hooks/useTheme.ts

Branch Name: improvement/theme-persistence

[Lansa-18]

Hi @Sendi0011, I'd like to work on this.
I'll set up localStorage persistence in theme-provider.tsx (with a check for prefers-color-scheme as the initial fallback), build a useTheme hook for reading/updating the theme, and add a theme-toggle.tsx component with smooth CSS transitions on theme change. I'll wire the toggle into header.tsx and make sure layout.tsx applies the saved theme on initial load to avoid a flash of the wrong theme.

Thanks.

[Meet-hybrid]

Can I work on this please?

[grantfox-oss]

🦊 GrantFox — @Lansa-18 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #4)
2. Your PR will be reviewed by the JointSave-org maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Lansa-18's PR #27 was approved and merged by @Sendi0011.

🏆 @Lansa-18: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (83 total points). Track your full progress on GrantFox.

👏 Great work, @Lansa-18! Keep contributing to JointSave-org.