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>

backend/app/core/config.py

@@ -5,7 +5,7 @@ from pydantic_settings import BaseSettings
 
 
 class Settings(BaseSettings):
-    PROJECT_NAME: str = "PragmaStack"
+    PROJECT_NAME: str = "Syndarix"
     VERSION: str = "1.0.0"
     API_V1_STR: str = "/api/v1"