Add dark/light theme toggle for the AI Chat panel

[FatBy]

Description

The AI Chat panel currently has a fixed dark theme. Add an option to toggle between dark and light themes for the chat area.

Current Behavior

• Chat panel always uses dark background
• No user preference for chat theme

Expected Behavior

• A toggle button in the chat panel header
• Light theme with readable contrast
• Preference saved to localStorage

Files to Modify

• src/components/ai/AIChatPanel.tsx
• Possibly create a theme context or use existing Zustand store

Notes

The World View already has theme switching (Cosmos/City/Minimalist). This issue is specifically about the chat panel's color scheme.