feat: implement light/dark theme toggle

[pharwasz]

Closes #25

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Summary

This PR implements a complete light/dark theme system for the Wraith website while preserving the existing dark theme experience.

Changes

• Added a light theme using CSS custom properties with :root[data-theme="light"]
• Refactored theme colors to use CSS variables for both themes
• Added a theme toggle to the header (desktop and mobile)
• Persisted the selected theme using localStorage
• Defaulted to the user's system preference via prefers-color-scheme
• Added an inline initialization script to prevent first-paint theme flashing
• Added smooth 200ms transitions when switching themes
• Updated the browser theme-color dynamically to match the active theme
• Improved accessibility with descriptive aria-label, aria-pressed, and title attributes on the theme toggle
• Documented the light theme design in design/wraith.pen

Testing

• Theme toggles correctly between light and dark modes
• Theme preference persists after page refresh
• Defaults to prefers-color-scheme when no saved preference exists
• No flash of the incorrect theme on initial page load
• Desktop header toggle works correctly
• Mobile menu theme toggle works correctly
• Project builds successfully (pnpm build)

Screenshots

Dark Theme

Light Theme

Notes

This implementation follows the issue requirements by:

• Using CSS variables for all theme colors
• Supporting both dark and light themes
• Persisting the user's preference
• Respecting system theme preferences
• Preventing first-paint theme flashing
• Maintaining smooth theme transitions
• Improving accessibility of the theme toggle

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[truthixify]

Clean merge. Light/dark theme toggle in Header + CSS vars + pencil file update. Thanks @pharwasz.