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/shared/js/api-client.js

@@ -59,6 +59,9 @@ class APIClient {
         } else if (currentPath.includes('/shop/') || currentPath.startsWith('/api/v1/shop/')) {
             token = customerToken;
             source = 'customer (path-based)';
+        } else if (currentPath.startsWith('/merchants/') || currentPath.startsWith('/api/v1/merchants/')) {
+            token = localStorage.getItem('merchant_token');
+            source = 'merchant (path-based)';
         } else {
             // Default fallback for other paths
             token = adminToken || storeToken || customerToken;
@@ -382,6 +385,9 @@ class APIClient {
         } else if (currentPath.includes('/shop/') || currentPath.startsWith('/api/v1/shop/')) {
             apiLog.info('Clearing customer tokens only');
             localStorage.removeItem('customer_token');
+        } else if (currentPath.startsWith('/merchants/') || currentPath.startsWith('/api/v1/merchants/')) {
+            apiLog.info('Clearing merchant tokens only');
+            localStorage.removeItem('merchant_token');
         } else {
             // Fallback: clear all tokens for unknown paths
             apiLog.info('Unknown path context, clearing all tokens');