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>

Makefile

@@ -399,26 +399,22 @@ tailwind-install:
 	@mv tailwindcss-linux-x64 $(TAILWIND_CLI)
 	@echo "Tailwind CLI installed: $$($(TAILWIND_CLI) --help | head -1)"
 
+# All frontends that have a Tailwind build (static/<name>/css/tailwind.css)
+TAILWIND_FRONTENDS := admin store storefront platform merchant
+
 tailwind-dev:
 	@echo "Building Tailwind CSS (development)..."
-	$(TAILWIND_CLI) -i static/admin/css/tailwind.css -o static/admin/css/tailwind.output.css
-	$(TAILWIND_CLI) -i static/store/css/tailwind.css -o static/store/css/tailwind.output.css
-	$(TAILWIND_CLI) -i static/shop/css/tailwind.css -o static/shop/css/tailwind.output.css
-	$(TAILWIND_CLI) -i static/platform/css/tailwind.css -o static/platform/css/tailwind.output.css
-	@echo "Tailwind CSS built (admin + store + shop + platform)"
+	@$(foreach fe,$(TAILWIND_FRONTENDS),$(TAILWIND_CLI) -i static/$(fe)/css/tailwind.css -o static/$(fe)/css/tailwind.output.css &&) true
+	@echo "Tailwind CSS built ($(TAILWIND_FRONTENDS))"
 
 tailwind-build:
 	@echo "Building Tailwind CSS (production - minified)..."
-	$(TAILWIND_CLI) -i static/admin/css/tailwind.css -o static/admin/css/tailwind.output.css --minify
-	$(TAILWIND_CLI) -i static/store/css/tailwind.css -o static/store/css/tailwind.output.css --minify
-	$(TAILWIND_CLI) -i static/shop/css/tailwind.css -o static/shop/css/tailwind.output.css --minify
-	$(TAILWIND_CLI) -i static/platform/css/tailwind.css -o static/platform/css/tailwind.output.css --minify
+	@$(foreach fe,$(TAILWIND_FRONTENDS),$(TAILWIND_CLI) -i static/$(fe)/css/tailwind.css -o static/$(fe)/css/tailwind.output.css --minify &&) true
 	@echo "Tailwind CSS built and minified for production"
 
 tailwind-watch:
-	@echo "Watching Tailwind CSS for changes..."
-	@echo "Note: This watches admin CSS only. Run in separate terminal."
-	$(TAILWIND_CLI) -i static/admin/css/tailwind.css -o static/admin/css/tailwind.output.css --watch
+	@echo "Watching Tailwind CSS for changes ($(fe))..."
+	$(TAILWIND_CLI) -i static/$(fe)/css/tailwind.css -o static/$(fe)/css/tailwind.output.css --watch
 
 # =============================================================================
 # CELERY / TASK QUEUE
@@ -593,7 +589,7 @@ help:
 	@echo "  tailwind-install     - Install Tailwind standalone CLI"
 	@echo "  tailwind-dev         - Build Tailwind CSS (development)"
 	@echo "  tailwind-build       - Build Tailwind CSS (production, minified)"
-	@echo "  tailwind-watch       - Watch and rebuild on changes"
+	@echo "  tailwind-watch fe=X  - Watch and rebuild on changes (specify frontend)"
 	@echo ""
 	@echo "=== CELERY / TASK QUEUE ==="
 	@echo "  celery-worker        - Start Celery worker"