feat(theme): add persistent dark mode toggle

[lovesmilesmall-hue]

Summary

Adds a light/dark theme toggle to the frontend with a persistent user preference. Users can switch themes from the navbar, the choice is remembered across reloads, and first-time visitors get the theme matching their system preference.

Problem

Tailwind was configured for class-based dark mode (darkMode: ["class"]), but nothing ever added the dark class to the document, so the dark styles were unreachable. There was no control to switch themes, no persistence of a user's choice, and no handling of the system preference, meaning the app always rendered in light mode regardless of the environment.

Changes Made

• Added frontend/hooks/useTheme.js: resolves the active theme from the persisted localStorage value, falling back to prefers-color-scheme, applies the dark/light class to document.documentElement, and persists an explicit toggle choice under stellarhunts.theme.
• Added frontend/components/ThemeToggle.jsx: a button that switches between themes and shows a sun or moon icon for the current theme.
• Wired the toggle into frontend/components/Navbar.jsx next to the mobile menu control.
• Added an inline script in frontend/app/layout.js that applies the stored or system theme before hydration to avoid a light-mode flash on load.

Implementation Details

useTheme toggles the dark/light classes on the root element, matching the existing Tailwind class strategy so all components using dark: variants are themed consistently. The initial class is set by a small inline script in the document head, which reads localStorage first and otherwise consults matchMedia("(prefers-color-scheme: dark)"); React state then syncs to that value on mount so the toggle reflects reality without a flash.

Closes #692

Closes #694

Closes #695

Closes #706

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[dzekojohn4]

LGTM