Add Supabase key validation and simplify frontend state management

- Add backend validation to detect and warn about anon vs service keys
- Prevent startup with incorrect Supabase key configuration
- Consolidate frontend state management following KISS principles
- Remove duplicate state tracking and sessionStorage polling
- Add clear error display when backend fails to start
- Improve .env.example documentation with detailed key selection guide
- Add comprehensive test coverage for validation logic
- Remove unused test results checking to eliminate 404 errors

The implementation now warns users about key misconfiguration while
maintaining backward compatibility. Frontend state is simplified with
MainLayout as the single source of truth for backend status.

python/tests/test_settings_api.py

@@ -56,3 +56,5 @@ def test_existing_credential_returns_normally(client, mock_supabase_client):
         assert data["is_encrypted"] is False
         # Should not have is_default flag for real credentials
         assert "is_default" not in data
+
+