feat(frontend): Add dark mode toggle with system preference detection

[KuchiMercy]

Description

ThemeProvider exists but dark mode not fully implemented. Audit all components and implement dark mode with system preference detection.

Provider: ThemeProvider.tsx | Styling: Tailwind CSS v4

Acceptance Criteria

• Audit every component for dark mode compatibility
• Add dark: variant classes where missing
• Dark mode on all pages, modals, forms, skeletons, toasts, badges, navigation
• Theme toggle button (sun/moon icon) in navigation header
• Detect system preference via prefers-color-scheme on first visit
• Persist preference in localStorage (vaultix-theme)
• Persisted preference overrides system preference
• Three modes: Light, Dark, System
• Smooth transitions (no flash)
• All ShadCN/Radix components correct in dark mode
• Visual regression tests (Playwright screenshots)

Points: 100

[abimbolaalabi]

@abimbolaalabi has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer! I’m interested in contributing to this task and can ensure timely delivery with clean implementation. I’ve thoroughly read the issue description and understand the expected outcome. I’d love to work on it. Thank You!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @abimbolaalabi to this issue.

[drips-wave]

Congratulations, @abimbolaalabi! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @abimbolaalabi: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @abimbolaalabi as part of the Stellar Wave Program's 6th Wave 🥳

😎 @abimbolaalabi: You earned 100 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.