feat(shop): add product dialog popup on products page

[aris-cub]

Summary

• New ProductDialog component — modal overlay with image gallery (front/back toggle), product description, price, stock info, and an "Add to cart" button
• Updated /products page — clicking a product tile now opens the dialog inline instead of navigating to /products/[id]
• Added dialog animations — smooth fadeIn backdrop and slideUp panel entrance in globals.css

The dialog supports keyboard dismiss (Escape), click-outside-to-close, scroll lock while open, and is fully accessible (role="dialog", aria-modal).

Test plan

• Navigate to /products and click a product tile — dialog should appear with product details
• Verify image gallery thumbnails toggle between front/back images
• Click "Add to cart" in the dialog — should navigate to cart with the product added
• Press Escape or click the backdrop — dialog should close
• Confirm the close button (X) in the top-right corner works
• Verify the page behind the dialog does not scroll while the dialog is open
• Test on mobile viewport — dialog should be responsive

https://claude.ai/code/session_013LJXFRo7A4PdPgcbA9Dk7n

[github-actions]

mirrord Preview Environment - Metal Mart

Preview environments are running for this PR.

Service	Target
metal-mart-frontend	deployment/metal-mart-frontend

Preview URL	https://playground.metalbear.dev/shop
Header	baggage: mirrord=claude-add-dialog-popup-hiOep

To send traffic to this preview:

• Use the mirrord Browser Extension and set the header for this URL, or
• curl -H "baggage: mirrord=claude-add-dialog-popup-hiOep" https://playground.metalbear.dev/shop

Preview is stopped when the PR is merged or closed.