Add dark/light theme persistence to localStorage

[Lubas1337]

Description

The Kanban web dashboard could benefit from a dark/light theme toggle that persists the user's preference to localStorage.

Goal

Add a theme toggle button to the dashboard header. Save the selected theme in localStorage so it persists across page reloads.

Technical hints

• Frontend: web/src/ — React with inline styles or CSS variables
• Consider using CSS custom properties (variables) for theming
• Check prefers-color-scheme media query for initial default

Acceptance criteria

• Toggle button switches between dark and light themes
• Theme preference saved in localStorage
• Respects system preference on first visit