[PDP-001] Product Detail Page: Full product view with images & specs

## Overview
Build the Product Detail Page (PDP) — a dedicated page for each product with a full image gallery (emoji placeholder for now), description, specs, price, and an Add to Cart button.

## Acceptance Criteria
- [ ] Route: `/product/:id` renders the correct product
- [ ] Displays: product name, category badge, price (with sale price if applicable), emoji image, short description, and key specs
- [ ] "Add to Cart" button calls the cart state module (#2)
- [ ] Breadcrumb navigation (Home > Category > Product)
- [ ] "Back to Products" link
- [ ] Product not found shows a 404 state
- [ ] Page title and meta description update dynamically for SEO

## Future Iterations (out of scope for Sprint 1)
- Real product images (asset pipeline not ready)
- Customer reviews section (#20)
- Related products / recommendations

## Depends On
- None — this is a standalone UI task

## Blocks
- #16 SEARCH-001 (search results link to this page)
- #20 REVIEW-001 (reviews are embedded on PDP)
- #23 PERF-001 (image optimization tied to PDP)