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>

app/core/frontend_detector.py

@@ -50,6 +50,7 @@ class FrontendDetector:
         "/api/v1/shop",  # Legacy support
         "/stores/",  # Path-based store access
     )
+    MERCHANT_PATH_PREFIXES = ("/merchants", "/api/v1/merchants")
     PLATFORM_PATH_PREFIXES = ("/api/v1/platform",)
 
     @classmethod
@@ -94,6 +95,10 @@ class FrontendDetector:
             logger.debug("[FRONTEND_DETECTOR] Detected ADMIN from path")
             return FrontendType.ADMIN
 
+        if cls._matches_any(path, cls.MERCHANT_PATH_PREFIXES):
+            logger.debug("[FRONTEND_DETECTOR] Detected MERCHANT from path")
+            return FrontendType.MERCHANT
+
         # Check storefront BEFORE store since /api/v1/storefront starts with /api/v1/store
         if cls._matches_any(path, cls.STOREFRONT_PATH_PREFIXES):
             logger.debug("[FRONTEND_DETECTOR] Detected STOREFRONT from path")