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/base.py

@@ -95,6 +95,7 @@ class MenuItemDefinition:
     requires_permission: str | None = None
     badge_source: str | None = None
     is_super_admin_only: bool = False
+    header_template: str | None = None  # Optional partial for custom header rendering
 
 
 @dataclass