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/lib/i18n/metadata.ts

@@ -13,8 +13,8 @@ export type Locale = 'en' | 'it';
  */
 export const siteConfig = {
   name: {
-    en: 'PragmaStack',
-    it: 'PragmaStack',
+    en: 'Syndarix',
+    it: 'Syndarix',
   },
   description: {
     en: 'Production-ready FastAPI + Next.js full-stack template with authentication, admin panel, and comprehensive testing',