Add dark/light theme toggle

[travisbreaks]

The frontend currently uses a fixed dark theme. Adding a theme toggle would improve usability.

Approach:

• Add a toggle button in the header
• Use CSS custom properties for theming
• Persist preference in localStorage
• Respect `prefers-color-scheme` as default

[urstrulypavan]

Hi @travisbreaks! I'd like to work on this. Could you assign it to me?