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>

static/storefront/css/tailwind.css

@@ -1,11 +1,11 @@
-/* Tailwind CSS v4 - Shop Frontend Styles */
+/* Tailwind CSS v4 - Storefront Styles */
 /* Configuration is CSS-first in v4 */
 
 @import "tailwindcss";
 
 /* Source paths for content scanning */
 @source "../../js/**/*.js";
-@source "../../../app/templates/shop/**/*.html";
+@source "../../../app/templates/storefront/**/*.html";
 @source "../../../app/templates/shared/**/*.html";
 
 /* Custom theme configuration */
@@ -198,7 +198,7 @@
   }
 }
 
-/* Shop-specific component styles */
+/* Storefront-specific component styles */
 @layer components {
   /* Form input styles */
   .form-input,
@@ -242,13 +242,13 @@
     @apply focus:ring-purple-400 focus:ring-offset-gray-800;
   }
 
-  /* Shop-specific button using CSS variable for store theming */
-  .btn-shop-primary {
+  /* Storefront-specific button using CSS variable for store theming */
+  .btn-storefront-primary {
     background-color: var(--color-primary, #7e3af2);
     @apply text-white px-4 py-2 rounded-lg font-medium transition-all;
   }
 
-  .btn-shop-primary:hover {
+  .btn-storefront-primary:hover {
     filter: brightness(0.9);
   }