refactor: rename shop to storefront for consistency

Rename all "shop" directories and references to "storefront" to match
the API and route naming convention already in use.

Renamed directories:
- app/templates/shop/ → app/templates/storefront/
- static/shop/ → static/storefront/
- app/templates/shared/macros/shop/ → .../macros/storefront/
- docs/frontend/shop/ → docs/frontend/storefront/

Renamed files:
- shop.css → storefront.css
- shop-layout.js → storefront-layout.js

Updated references in:
- app/routes/storefront_pages.py (21 template references)
- app/modules/cms/routes/pages/vendor.py
- app/templates/storefront/base.html (static paths)
- All storefront templates (extends/includes)
- docs/architecture/frontend-structure.md

This aligns the template/static naming with:
- Route file: storefront_pages.py
- API directory: app/api/v1/storefront/
- Module routes: */routes/api/storefront.py
- URL paths: /storefront/*

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/storefront/img/placeholder.svg

@@ -0,0 +1,6 @@
+<svg width="400" height="400" xmlns="http://www.w3.org/2000/svg">
+  <rect width="400" height="400" fill="#f3f4f6"/>
+  <text x="50%" y="50%" font-family="Arial, sans-serif" font-size="20" fill="#9ca3af" text-anchor="middle" dominant-baseline="middle">
+    No Image Available
+  </text>
+</svg>