fix(ci): add .gitkeep to SPA dist dir so test_spa_dist_dir_exists passes
Fixes test_dashboard_v2.py::test_spa_dist_dir_exists_as_placeholder. Directory must exist for package_data to include it; Docker build populates it via npm run build. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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