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>

app/templates/shop/home.html

@@ -24,7 +24,7 @@
             {{ vendor.description }}
         </p>
         {% endif %}
-        <a href="{{ base_url }}products" class="inline-block bg-white text-gray-900 px-8 py-3 rounded-lg font-semibold hover:bg-gray-100 transition">
+        <a href="{{ base_url }}shop/products" class="inline-block bg-white text-gray-900 px-8 py-3 rounded-lg font-semibold hover:bg-gray-100 transition">
             Shop Now
         </a>
     </div>
@@ -61,7 +61,7 @@
             <h2 class="text-3xl font-bold text-gray-800 dark:text-gray-200">
                 Featured Products
             </h2>
-            <a href="{{ base_url }}products" class="text-primary hover:underline font-medium">
+            <a href="{{ base_url }}shop/products" class="text-primary hover:underline font-medium">
                 View All →
             </a>
         </div>