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

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/README.md

@@ -1,4 +1,4 @@
-# PragmaStack - Frontend
+# Syndarix - Frontend
 
 Production-ready Next.js 16 frontend with TypeScript, authentication, admin panel, and internationalization.