Implement Phase 3: single product page templates (Bootstrap 5)

Add 21 Twig template overrides for the single product page:

Product layout:
- product-image: gallery with thumbnail strip, img-fluid rounded
- title: h1 entry-title
- price: fs-3 fw-bold with sale del/ins markup
- short-description: lead text-body-secondary
- meta: dl row with SKU, categories, tags
- rating: Bootstrap Icon stars with half-star, review count link
- stock: badge (bg-success/bg-danger/bg-warning) per status
- sale-flash: badge bg-danger fs-6
- share: hook-only wrapper
- product-attributes: table-sm table-striped

Related/upsells:
- related, up-sells: section with product loop grid

Tabs:
- tabs: nav-tabs + tab-content with fade transitions
- description: tab-pane with prose content
- additional-information: tab-pane with attributes hook

Add to cart (4 product types + variation JS):
- simple: input-group quantity + btn-primary btn-lg
- variable: form-select per attribute + variation display
- grouped: table-borderless with quantity per child product
- external: btn-outline-primary with external link icon
- variation: Underscore.js script templates (Bootstrap-styled)
- variation-add-to-cart-button: quantity + submit with hidden fields

CSS additions: gallery thumbnail hover, variation selector spacing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

PLAN.md

@@ -549,27 +549,27 @@ Track completion per file. Mark with `[x]` when done.
 
 ### Phase 3 -- Single Product
 
-- [ ] `single-product/product-image.html.twig`
-- [ ] `single-product/title.html.twig`
-- [ ] `single-product/price.html.twig`
-- [ ] `single-product/short-description.html.twig`
-- [ ] `single-product/meta.html.twig`
-- [ ] `single-product/rating.html.twig`
-- [ ] `single-product/stock.html.twig`
-- [ ] `single-product/sale-flash.html.twig`
-- [ ] `single-product/share.html.twig`
-- [ ] `single-product/product-attributes.html.twig`
-- [ ] `single-product/related.html.twig`
-- [ ] `single-product/up-sells.html.twig`
-- [ ] `single-product/add-to-cart/simple.html.twig`
-- [ ] `single-product/add-to-cart/variable.html.twig`
-- [ ] `single-product/add-to-cart/grouped.html.twig`
-- [ ] `single-product/add-to-cart/external.html.twig`
-- [ ] `single-product/add-to-cart/variation.html.twig`
-- [ ] `single-product/add-to-cart/variation-add-to-cart-button.html.twig`
-- [ ] `single-product/tabs/tabs.html.twig`
-- [ ] `single-product/tabs/description.html.twig`
-- [ ] `single-product/tabs/additional-information.html.twig`
+- [x] `single-product/product-image.html.twig`
+- [x] `single-product/title.html.twig`
+- [x] `single-product/price.html.twig`
+- [x] `single-product/short-description.html.twig`
+- [x] `single-product/meta.html.twig`
+- [x] `single-product/rating.html.twig`
+- [x] `single-product/stock.html.twig`
+- [x] `single-product/sale-flash.html.twig`
+- [x] `single-product/share.html.twig`
+- [x] `single-product/product-attributes.html.twig`
+- [x] `single-product/related.html.twig`
+- [x] `single-product/up-sells.html.twig`
+- [x] `single-product/add-to-cart/simple.html.twig`
+- [x] `single-product/add-to-cart/variable.html.twig`
+- [x] `single-product/add-to-cart/grouped.html.twig`
+- [x] `single-product/add-to-cart/external.html.twig`
+- [x] `single-product/add-to-cart/variation.html.twig`
+- [x] `single-product/add-to-cart/variation-add-to-cart-button.html.twig`
+- [x] `single-product/tabs/tabs.html.twig`
+- [x] `single-product/tabs/description.html.twig`
+- [x] `single-product/tabs/additional-information.html.twig`
 
 ### Phase 4 -- Cart