WEB3-003: Dark/Light Theme System

[FuRankLy]

User Story

As a user, I want to switch between dark and light mode so I can use the site comfortably in any lighting.

Description

Implement a global dark/light mode toggle. Should remember the user's preference across visits. Default to the OS setting if no preference is set. No flash of the wrong theme on page load.

Acceptance Criteria

• Toggle switch works and matches the design
• Preference persists across sessions
• Defaults to OS colour scheme
• No flash of wrong theme on load
• Every page and component respects the theme
• Accessible from any component

Points: 5 · Blocked By: Designer A's Design System (can stub with placeholders)