PDP-001: Product Detail Page — full product view with specs, breadcrumbs, and cart integration

## Overview
Create a dedicated product detail page (`product.html`) that shows full product information when a user clicks any product card. Route via URL param (`?id=N`).

## Acceptance Criteria
- [ ] Navigating to `product.html?id=1` renders the correct product
- [ ] Page includes: breadcrumb nav (Home > Category > Product), large emoji/image placeholder, product name, price, rating with review count, short description, and an Add to Cart button
- [ ] 404-style message shown for unknown product IDs
- [ ] Cart integration: Add to Cart works the same as on the catalog page
- [ ] Back link returns to the catalog (preserving the active category filter if possible)
- [ ] No new dependencies introduced; vanilla HTML/CSS/JS only

## Notes
This is a self-contained feature with no upstream dependencies — a great candidate for the Copilot coding agent.

Start with the catalog page's product cards and existing cart integration, then use product.html as the new entry point for the ?id=N route. Verify each acceptance criterion by opening a known product, an unknown ID, and the back link; done means the full product details render and Add to Cart behaves as it does on the catalog.

Written by the indexing model from the issue text.