feat(merchant): extract merchant portal as first-class frontend with auth, Tailwind fixes, and Gitea CI

- Extract login/dashboard from billing module into core (matching admin pattern)
- Add merchant auth API with path-isolated cookies (path=/merchants)
- Add merchant base layout with sidebar/header partials and Alpine.js init
- Add frontend detection and login redirect for MERCHANT type
- Wire merchant token in shared api-client.js (get/clear)
- Migrate billing templates to merchant base with dark mode support
- Fix Tailwind: rename shop→storefront in sources and config
- DRY Makefile tailwind targets with TAILWIND_FRONTENDS loop
- Rebuild all Tailwind outputs (production minified)
- Add Gitea Actions CI workflow (ruff, pytest, architecture, docs)
- Add Gitea deployment documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mkdocs.yml

@@ -160,7 +160,7 @@ nav:
       - Store Operations Expansion: development/migration/store-operations-expansion.md
 
     - Implementation Plans:
-      - Module Migration Plan: proposals/module-migration-plan.md
+      - Module Migration Plan: archive/module-migration-plan.md
       - Admin Inventory Management: implementation/inventory-admin-migration.md
       - Admin Notification System: implementation/admin-notification-system.md
       - Letzshop Order Import: implementation/letzshop-order-import-improvements.md
@@ -196,6 +196,7 @@ nav:
       - Docker: deployment/docker.md
       - CloudFlare Setup: deployment/cloudflare.md
       - GitLab CI/CD: deployment/gitlab.md
+      - Gitea CI/CD: deployment/gitea.md
       - Environment Variables: deployment/environment.md
       - Stripe Integration: deployment/stripe-integration.md