feat: implement vendor landing pages with multi-template support and fix shop routing

Major improvements to shop URL routing and vendor landing page system:

## Landing Page System
- Add template field to ContentPage model for flexible landing page designs
- Create 4 landing page templates: default, minimal, modern, and full
- Implement smart root handler to serve landing pages or redirect to shop
- Add create_landing_page.py script for easy landing page management
- Support both domain/subdomain and path-based vendor access
- Add comprehensive landing page documentation

## Route Fixes
- Fix duplicate /shop prefix in shop_pages.py routes
- Correct product detail page routing (was /shop/shop/products/{id})
- Update all shop routes to work with router prefix mounting
- Remove unused public vendor endpoints (/api/v1/public/vendors)

## Template Link Corrections
- Fix all shop template links to include /shop/ prefix
- Update breadcrumb 'Home' links to point to vendor root (landing page)
- Update header navigation 'Home' link to point to vendor root
- Correct CMS page links in footer navigation
- Fix account, cart, and error page navigation links

## Navigation Architecture
- Establish two-tier navigation: landing page (/) and shop (/shop/)
- Document complete navigation flow and URL hierarchy
- Support for vendors with or without landing pages (auto-redirect fallback)
- Consistent breadcrumb and header navigation behavior

## Documentation
- Add vendor-landing-pages.md feature documentation
- Add navigation-flow.md with complete URL hierarchy
- Update shop architecture docs with error handling section
- Add orphaned docs to mkdocs.yml navigation
- Document multi-access routing patterns

## Database
- Migration f68d8da5315a: add template field to content_pages table
- Support template values: default, minimal, modern, full

This establishes a complete landing page system allowing vendors to have
custom marketing homepages separate from their e-commerce shop, with
flexible template options and proper navigation hierarchy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

models/database/content_page.py

@@ -49,6 +49,11 @@ class ContentPage(Base):
     content = Column(Text, nullable=False)  # HTML or Markdown
     content_format = Column(String(20), default="html")  # html, markdown
 
+    # Template selection (for landing pages)
+    # Options: 'default', 'minimal', 'modern', 'full'
+    # Only used for landing pages (slug='landing' or 'home')
+    template = Column(String(50), default="default", nullable=False)
+
     # SEO
     meta_description = Column(String(300), nullable=True)
     meta_keywords = Column(String(300), nullable=True)
@@ -110,6 +115,7 @@ class ContentPage(Base):
             "title": self.title,
             "content": self.content,
             "content_format": self.content_format,
+            "template": self.template,
             "meta_description": self.meta_description,
             "meta_keywords": self.meta_keywords,
             "is_published": self.is_published,