feat: comprehensive vendor settings overhaul with Company inheritance

- Add structured API response with business_info, localization, letzshop,
  invoice_settings, theme_settings, domains, and stripe_info sections
- Add PUT /vendor/settings/business-info with reset_to_company capability
- Add PUT /vendor/settings/letzshop with validation for tax rates, delivery
- Add 9 settings sections: General, Business Info, Localization, Marketplace,
  Invoices, Branding, Domains, API & Payments, Notifications
- Business Info shows "Inherited" badges and Reset buttons for company fields
- Marketplace section includes Letzshop CSV URLs and feed options
- Read-only sections for Invoices, Branding, Domains, API with contact support
- Add globe-alt icon for Domains section
- Document email templates architecture for future implementation

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -168,6 +168,7 @@ nav:
       - OMS Feature Plan: implementation/oms-feature-plan.md
       - Vendor Frontend Parity: implementation/vendor-frontend-parity-plan.md
       - Stock Management Integration: implementation/stock-management-integration.md
+      - Email Templates Architecture: implementation/email-templates-architecture.md
 
     # --- Testing ---
     - Testing: