Root cause: CSS and other static assets are not being served - Flask needs static file serving configuration
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Evidence: curl -s -o /dev/null -w "%{http_code}" http://localhost:8000/style.css returns 200.
Fix in place: api_routes/static_assets.py has a @router.get("/style.css") handler (lines 58-68) that serves site/style.css with an inline fallback. The router is imported and registered in api.py at lines 891 and 896 via app.include_router(_static_router).
Relevant commits: 27a243752 ([Atlas] Fix static asset serving: CSS/JS/notebooks fallback to FastAPI), 2e74bc765 ([Atlas] api.py: auto-restore site/style.css from git on startup if missing from working tree)