[FE-37] Add dark mode support with Tailwind dark class strategy

[yusuftomilola]

Overview

Add dark mode support to the app using Tailwind CSS dark class strategy and a user preference toggle inside frontend/contrib/.

Location

All work must be inside frontend/contrib/.

Acceptance Criteria

• Tailwind configured with darkMode: "class" in tailwind.config.ts
• Theme preference stored in localStorage and applied on initial load with no flash
• useTheme() hook exposes theme, toggleTheme(), and setTheme("light" | "dark")
• Theme toggle button added to the topbar (sun/moon Lucide icon)
• All existing UI components have dark: variants applied
• System prefers-color-scheme used as default when no stored preference exists

[Dydex]

@Dydex has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi Maintainer

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