Fix dashboard first load latency

[cbusillo]

Summary

• render the Work page shell immediately instead of showing the route loading wall
• split the dashboard core fast path from folder hydration with preview_limit=0
• skip series-folder hydration for the Work page and keep it on the Folders index
• add regression coverage that preview_limit=0 skips folder-card generation while keeping catalog emptiness truthful

Validation

• browser reload at http://127.0.0.1:4173/: no Loading workstation data, Work shell paints immediately, folder rows hydrate afterward
• live API timings: /api/dashboard?preview_limit=0 ~162 ms / 47 KB; /api/dashboard/folders?include_series=0 hydrates in background
• uv run --with pytest pytest tests/test_tuning_runtime.py -q -k dashboard_summary_payload
• npm --prefix frontend run check
• npm --prefix frontend run test:unit -- --run
• bash scripts/pre-commit-check.sh