Add dark mode support

[grmbyrn]

Problem

The app has no dark mode. Given the target audience skews developer,
dark mode is a frequently expected feature and Tailwind v4 makes it
straightforward to implement.

Proposed Solution

Implement dark mode using Tailwind's dark: variant and a theme toggle
stored in localStorage or a cookie.

Acceptance Criteria

• All pages and components have dark: variants for backgrounds, text, and borders
• A theme toggle button is accessible from the nav
• User preference is persisted across sessions
• Defaults to the OS preference via prefers-color-scheme

Labels

enhancement