✨ Add manifest.json for PWA support

[fusion94]

Description

A web app manifest enables PWA features including 'Add to Home Screen' on mobile, custom app icons, and splash screens. Currently missing from the site.

Implementation

Create manifest.json:

{
  "name": "Collectors Playbook",
  "short_name": "CP Cards",
  "description": "Premium sports cards marketplace",
  "start_url": "/",
  "display": "standalone",
  "background_color": "#1B2951",
  "theme_color": "#F4B643",
  "icons": [...]
}

Add to all pages:

<link rel="manifest" href="/manifest.json">

Acceptance Criteria

• manifest.json created with brand colors and icons
• Linked from all HTML pages
• Passes Chrome PWA audit