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/core/services/menu_discovery_service.py

@@ -67,6 +67,7 @@ class DiscoveredMenuItem:
     section_order: int
     is_visible: bool = True
     is_module_enabled: bool = True
+    header_template: str | None = None
 
 
 @dataclass
@@ -191,6 +192,7 @@ class MenuDiscoveryService:
                         section_label_key=section.label_key,
                         section_order=section.order,
                         is_module_enabled=is_module_enabled,
+                        header_template=item.header_template,
                     )
                     sections_map[section.id].items.append(discovered_item)