Fix renderer list and enable markdown tables

- Fix get_user_renderers usage (returns strings not dicts)
- Enable tables and fenced_code markdown extensions

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

app/routers/users.py

@@ -149,7 +149,7 @@ async def home(request: Request):
         from pathlib import Path
         readme_path = Path(__file__).parent.parent.parent / "README.md"
         if readme_path.exists():
-            readme_html = markdown.markdown(readme_path.read_text())
+            readme_html = markdown.markdown(readme_path.read_text(), extensions=['tables', 'fenced_code'])
     except Exception:
         pass