feat: complete Syndarix rebranding from PragmaStack

- Update PROJECT_NAME to Syndarix in backend config
- Update all frontend components with Syndarix branding
- Replace all GitHub URLs with Gitea Syndarix repo URLs
- Update metadata, headers, footers with new branding
- Update tests to match new URLs
- Update E2E tests for new repo references
- Preserve "Built on PragmaStack" attribution in docs

Closes #13

frontend/src/app/[locale]/admin/layout.tsx

@@ -12,7 +12,7 @@ import { AdminSidebar, Breadcrumbs } from '@/components/admin';
 
 export const metadata: Metadata = {
   title: {
-    template: '%s | Admin | PragmaStack',
+    template: '%s | Admin | Syndarix',
     default: 'Admin Dashboard',
   },
 };