Feature: Brand-aware style generation from URL or CSS variables

Idea: What if Phase 2 could accept a brand source (a URL or a CSS file) and generate style previews that match an existing brand?

Why: The most common slide use case for teams is decks that match their product. Right now you'd manually copy hex codes into a preset. A brand import would let you point at your site and get 3 layout variations that already look like your brand.

How it could work:

1. In Phase 1, new question: "Do you have a brand to match?" → URL / CSS file / No
2. If URL: fetch the page, extract :root CSS variables, dominant colors, font-family declarations from stylesheets
3. If CSS file: parse variables directly
4. In Phase 2: use the extracted palette and fonts as the foundation for all 3 style previews — same brand, different layouts

The preset structure already supports this — each preset in STYLE_PRESETS.md is a :root block + font choices + signature elements. A brand import would just be a dynamically generated preset.

Scope: Could start minimal — just extract --color-* or --brand-* CSS variables + the first font-family declaration from a URL's stylesheet. That covers 80% of brand matching without needing to parse full design systems.