Add dark mode to dashboard

## Description
Add dark mode support to the CronPulse dashboard.

## Details
The dashboard is currently light-mode only. Add a toggle for dark mode that:
- Respects system preference by default (`prefers-color-scheme`)
- Allows manual toggle
- Persists preference in localStorage

## Notes
The dashboard uses SSR with Hono JSX and inline CSS. Dark mode can be implemented with CSS custom properties.