feat(blocks): add image/gallery/video, product list, and cart blocks

[mayeedwin]

Summary

Adds five new content blocks to the Conversed AST, plus a native <dialog> lightbox and CSS scroll-snap for horizontal strips. Ships in @conversed/core, @conversed/react, and @conversed/angular.

New blocks

• image — single image with optional aspect ratio; href opens a modal preview
• gallery — horizontal snap-scroll (default) or responsive grid of images; per-item click-through opens the shared modal
• video — HTML5 <video> with poster, aspect, and native controls
• product / products — full vertical ecommerce card (image on top, title, subtitle, rating, price + strike-through, badge, CTAs); wrap multiple in <section data-products> for a horizontal scroll strip or data-layout=\"grid\"
• cart — line items with thumbnail/qty/note, summary rows (with emphasized total), and checkout CTAs

Modal preview

• Native <dialog> lightbox with prev/next navigation, ArrowLeft/ArrowRight keys, click-to-zoom, ESC and backdrop-to-close
• Sized to min(70vw, 820px) at rest; expands to 92vw when zoomed to native size

Polish

• Horizontal strips (gallery, products) hide scrollbar chrome and use scroll-snap-type: x mandatory + scroll-behavior: smooth
• System prompt spec (CONVERSED_SYSTEM_PROMPT) updated with the new block shapes so LLMs can emit them
• Four new demo presets: Field gallery, Shop the store, Deals under $50, Order history

Why

The existing media block was a single-image blob with no distinction for video, no gallery, and no ecommerce shape. Agents commonly emit product listings, cart summaries, image galleries, and short walkthrough clips — landing these before 0.0.1 keeps the AST complete on release.

Reviewer notes

• MediaBlock (the pre-existing {src,alt,caption} shape from <figure><img>) is untouched for backward compatibility — the new ImageBlock opts in via data-image, data-aspect, or data-href on the <figure>
• Parser adds ~15 new reserved data-attrs; product/cart use idiomatic HTML (<article data-product>, <section data-cart>, <ul data-items>, <ul data-summary>) so payloads stay readable
• Angular ConversedGalleryComponent uses @ViewChild('dlg') for the shared modal because template refs can't cross @if/@for sibling boundaries
• Parser tests cover the new blocks (24/24 passing); library builds green for core, react, and angular

Test plan

• Trigger "Field gallery" preset → hero image opens modal, prev/next steps through Recent scans, video plays inline
• Trigger "Shop the store" → 4-product horizontal scroll snaps between cards, cart renders with 2 lines + 3 summary rows + Checkout/Continue shopping
• Follow-up chips "Show me deals under $50" and "What's in my order history?" resolve to their own presets
• Modal close (×, ESC, click backdrop) all clear the .zoomed state
• Confirm no horizontal page scroll on mobile viewport (375px)

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[mayeedwin]

LGTM