feat(storefront): section-based homepages, header action partials, fixes

Phase 1 — Section-based store homepages:
- Store defaults use template="full" with per-platform sections JSON
- OMS: shop hero + features + CTA; Loyalty: rewards hero + features + CTA
- Hosting: services hero + features + CTA
- Deep placeholder resolution for {{store_name}} inside sections JSON
- landing-full.html uses resolved page_sections from context

Phase 2 — Module-contributed header actions:
- header_template field on MenuItemDefinition + DiscoveredMenuItem
- Catalog provides header-search.html partial
- Cart provides header-cart.html partial with badge
- Base template iterates storefront_nav.actions with {% include %}
- Generic icon fallback for actions without a template

Fixes:
- Store theme API: get_store_by_code → get_store_by_code_or_subdomain

Docs:
- CMS redesign proposal: menu restructure, page types, translations UI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/cms/routes/pages/storefront.py

@@ -60,12 +60,14 @@ async def storefront_homepage(
         include_unpublished=False,
     )
 
-    # Resolve placeholders for store default pages (title + content)
+    # Resolve placeholders for store default pages (title, content, sections)
     page_content = None
     page_title = None
+    page_sections = None
     if page:
         page_content = page.content
         page_title = page.title
+        page_sections = page.sections
         if page.is_store_default and store:
             page_content = content_page_service.resolve_placeholders(
                 page.content, store
@@ -73,12 +75,18 @@ async def storefront_homepage(
             page_title = content_page_service.resolve_placeholders(
                 page.title, store
             )
+            if page_sections:
+                page_sections = content_page_service.resolve_placeholders_deep(
+                    page_sections, store
+                )
 
     context = get_storefront_context(request, db=db, page=page)
     if page_content:
         context["page_content"] = page_content
     if page_title:
         context["page_title"] = page_title
+    if page_sections:
+        context["page_sections"] = page_sections
 
     # Select template based on page.template field (or default)
     template_map = {