Add individual background demo/docs pages

[alvinreal]

Summary

Each background should have its own page at /backgrounds/[id] with:

• Full-screen preview
• Configuration options / props panel
• Copy-paste code snippet
• Attribution info

Requirements

• Dynamic route: /backgrounds/[id]
• Props editor sidebar (sliders, color pickers where applicable)
• Code snippet with copy button
• Responsive layout
• Link from main gallery

Context

Currently users can only see backgrounds in the main carousel. Individual pages would make it much easier to evaluate and integrate specific backgrounds.