feat: add mandatory vendor onboarding wizard

Implement 4-step onboarding flow for new vendors after signup:
- Step 1: Company profile setup
- Step 2: Letzshop API configuration with connection testing
- Step 3: Product & order import CSV URL configuration
- Step 4: Historical order sync with progress bar

Key features:
- Blocks dashboard access until completed
- Step indicators with visual progress
- Resume capability (progress persisted in DB)
- Admin skip capability for support cases

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

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

mkdocs.yml

@@ -200,6 +200,7 @@ nav:
         - Implementation Guide: features/cms-implementation-guide.md
       - Platform Homepage: features/platform-homepage.md
       - Vendor Landing Pages: features/vendor-landing-pages.md
+      - Vendor Onboarding: features/vendor-onboarding.md
       - Subscription & Billing: features/subscription-billing.md
       - Email System: features/email-system.md