feat(showcase): implement e-commerce product card block in playground

[miguelramos]

Add the T3 composition block with two product cards (slate and mauve base schemas) demonstrating multi-schema coexistence. Features:

• 2-column responsive grid (stacks on mobile)
• Product images from picsum.photos with SVG fallback on error
• Price badge using accent schema (--line-solid-background)
• Rating stars using L3 alias --line-warning (amber) independent of schema
• Size selector chips with accent-highlighted active state
• Color selector dots using raw palette tokens (crimson, violet, slate, etc.)
• Add to Cart button reactive to nav schema picker accent
• Card neutrals use raw palette tokens (slate/mauve) directly in shadow DOM
• All accent-responsive elements inherit CSS custom properties through shadow DOM boundary from body's active schema

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Latest commit: 091b06d

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