[FEATURE] Add light/dark theme toggle

[ascender1729]

Description

Implement a theme toggle to switch between light and dark color modes.

Why Needed

• Reduces eye strain in different lighting
• User preference for appearance
• Modern web application standard

Implementation Details

• Create theme context/store for global state
• Detect system preference (prefers-color-scheme)
• Persist choice in localStorage
• Use CSS custom properties for theming
• Toggle button in header

Files to Create/Modify

• \ (new - theme store)
• \ (new)
• \ (new - CSS variables)

Code Example

\

Acceptance Criteria

• Toggle switches between light/dark
• Respects system preference on first visit
• Choice persists across sessions
• Smooth transition between themes
• All components themed consistently

Time Estimates

• With AI: 135 minutes
• Without AI: 900 minutes