feat: add system-aware theme and homepage redesign

[samholmes]

Summary

• add system-aware light and dark color tokens across pages, forms, cards, navigation, and shared components
• redesign the homepage around the potion background, rotating DEVx location wordmark, and full-width image accordion
• update shared buttons, cards, header navigation, and calls to action for the new responsive layout

Testing

• bunx tsc --noEmit
• bun run lint (passes with two existing warnings in app/layout.tsx)
• bun run build
• checked the homepage in desktop and mobile browser viewports